I am trying to understand how cover may or may not help against AOE spells. From what I understand from the rules, total cover affects the line of effect check to determines whether the target is affected by the spell at all. Then, if the spell requires a DEX save, depending on which direction(s) the effect reaches the target, the target gets the minimum DEX bonus from the maximum DEX bonus of each direction. When the line of effect can bend infinitely many times, unless the target has Total Cover within range from all sides, the line of effect could eventually reach the target from an opening.
Then:
1. A cover that wraps around the entire body could provide cover against spells that have a bending line of effect.
Example: A familiar that is in a pocket or within the armor of another character does not need to roll for DEX save against Fireball when it is totally covered from all sides.
2. Walls and Doors provide total cover for spells originating from the other side.
Example: A caster can't cast Antilife Shell from the other side of the door to prevent other characters from reaching for the doorknob because the door blocks the magic. A character in full plate armor cannot enter the Antilife Shell even with an opague visor because typically the armors and shields of a character are parts of the character "creature". However, a big sack can be dropped onto and enclose the caster, then the Antilife Shell can only extend to the interior wall of the bag, and other characters can all approach and tickle the caster through the bag. If the caster casts Antimagic Field, the Field would reach up to the bag including the bag. The bag itself can no longer be affected by telekinesis because the bag cannot provide Total Cover for itself. To telekinesis the bagged caster, another bag would need to enclose the bagged mage.
3. Obstructing the origin of an AOE effectively cancels the effect of the AOE (unless the spell specifically says otherwise).
Example: Sleet Storm has an origin at the bottom face of its cylinder. After the spell is cast, if someone places a shield or sits at the origin, the floor remains icy but the heavily obscuring storm is gone everywhere else besides the origin. After Silence is cast, a character could walk to the center, thus its own body is blocking the origin of the spell. A creature cannot provide Total Cover for itself, so the character obstructing the origin of the Silence spell is silenced, but other characters in the sphere become unaffected.
4. A character can ready Dash action to dash out of AOE range, to dash behind cover, or to dash to provide cover.
Example: Dashing from physical attack means that as the character sees the dragon flying to them, they dash away hoping to continually keep away from the dragon's physical reach. In these cases the trigger of the dash action is the movement of the dragon. The character would need to know where the dragon is. The trigger cannot be "when the dragon attacks", because that will be too late.
5. Total Cover can block dragon breath attacks.
Example: Depending on positioning and size, Floating Disk, object under Telekinesis, and creatures can all provide Total Cover. PHB gives an example that a creature can provide half cover. That is not a rule that a creature can ONLY provide half cover. A creature can also provide Total Cover depends on the size and positioning. A character could ready a dash action to continually position itself behind Total Cover or to provide Total Cover to another character, when the dragon moves. The trigger of the dash action needs to be the dragon's movement. When the dragon attacks, it is too late for the reaction to dash and provide cover. If a character is providing Total Cover to another character, regardless whether the Cover succeed or fail the CON save, the covered character is not affected by the breath attack.
I think you may be overthinking some of those, or you may be looking at the rules a little too closely (to the extend of considering semantics). Cover rules only mention examples because it's impossible to list every situation and the cover bonus it would include, so they're just guidelines so that the DM can make appropriate calls.
I like the breakdown, though, so I thought I'd give you my personal opinion (and potential handling) if such situations (and/or attempts) came up to one of my games, if that helps. :)
1. Sure, I don't think I'd kill the healer's celestial hamster companion if she always keeps it in a pocket or something. Rather than go with a bonus to save, I'd give it total cover unless it was doing something specific when catastrophe struck. I'd possibly consider the circumstances more carefully if it was a character that was miniaturized somehow and hid in a pocket, but I doubt I would, unless it was an attempt to exploit it.
2. Antilife Shell versus door? Interesting, and I'd likely err towards the players' ideas the first time it came up (and keep consistent with that handling thereafter). Sack providing total cover is too far, though - a sack isn't an obstacle any more than a character's clothes are, and it's not going to stop an Antilife Shell any more than it's going to stop a Silence spell or a Fireball. Trying to cover yourself up with a cloak to stop the damage from a cave-in is an inside joke to my group, but it didn't help the first time it was tried either. :p
3. Sure, but providing total cover isn't as simple as you seem to suggest (or infer by the rules), and is much less so when trying to figure out a point of origin of an effect (such as the center of a Silence spell. It's one thing to consider situations out of combat, where you can put your hand in front of a magical line of green light to stop it from shining on that emerald, but quite another to try to fully block an area of effect with your body in combat. You do not fill your space, so you technically never provide total cover in combat, other than the DM judging for dramatic appropriateness that jumping on the glowing orb of a Delayed Blast Fireball will stop it from affecting your companions (and will likely obliterate you).
4. Yes! If you can figure out a good trigger (perhaps the dragon scrounges up his nose every time he's about to breathe fire?), and the opponent isn't smart enough to notice what you're doing and bluff you (some dragons can be stupid!), this is a viable tactic. You do usually lose your action (and your reaction) to fully utilize such a defensive Ready, though, so it's hardly an efficient course of action - unless you have means to help a lot with Bonus Actions or similar, such as Healing Word or a Thief's Cunning Action. Or a Fighter's Action Surge - that's always good for a last ditch attempt to do something stylish. :p
5. Sure, but see 3. A Wall of Force (or other solid walls) is ideal for such things. A cloth bag isn't. :p
1. A cover that wraps around the entire body could provide cover against spells that have a bending line of effect.
Example: A familiar that is in a pocket or within the armor of another character does not need to roll for DEX save against Fireball when it is totally covered from all sides.
Solid armor could arguably provide cover to a small familiar, but you'd be hard-pressed to find a gap in armor where a familiar could fit and also not be crushed to death. Clothing isn't a solid obstacle and provides no cover. If you want to keep your familiar safe, dismiss it.
2. Walls and Doors provide total cover for spells originating from the other side.
... However, a big sack can be dropped onto and enclose the caster, then the Antilife Shell can only extend to the interior wall of the bag, and other characters can all approach and tickle the caster through the bag...
A bag isn't a solid obstacle and isn't damage-proof enough to provide total cover.
3. Obstructing the origin of an AOE effectively cancels the effect of the AOE (unless the spell specifically says otherwise).
Example: Sleet Storm has an origin at the bottom face of its cylinder. After the spell is cast, if someone places a shield or sits at the origin, the floor remains icy but the heavily obscuring storm is gone everywhere else besides the origin. After Silence is cast, a character could walk to the center, thus its own body is blocking the origin of the spell.
Nope. The effect for an area spell radiates out from the point of origin at the moment it was cast. Once the spell effect has been produced, the point of origin becomes irrelevant.
Also keep in mind the point of origin for a cylinder can be at the height of the spell, and the point of origin for a cube can be anywhere on its surface.
4. A character can ready Dash action to dash out of AOE range, to dash behind cover, or to dash to provide cover.
Example: Dashing from physical attack means that as the character sees the dragon flying to them, they dash away hoping to continually keep away from the dragon's physical reach. In these cases the trigger of the dash action is the movement of the dragon. The character would need to know where the dragon is. The trigger cannot be "when the dragon attacks", because that will be too late.
You could ready an action to move out of the way if an area effect is preceded by a perceptible circumstance, but this is rarely practical; if the area effect doesn't happen, you've wasted your action. Taking the Dodge is usually a better idea, and it also protects you against attacks.
Readying the Dash action arguably doesn't do anything, since it merely adds movement but you can't move when it's not your turn.
5. Total Cover can block dragon breath attacks.
Example: Depending on positioning and size, Floating Disk, object under Telekinesis, and creatures can all provide Total Cover.
Tenser's Floating Disk generally won't be of much help as total cover unless you can predict that something is going to fall on you.
PHB gives an example that a creature can provide half cover. That is not a rule that a creature can ONLY provide half cover. A creature can also provide Total Cover depends on the size and positioning.
A creature only provides half cover, regardless of size. Unless it's a gelatinous cube, most creatures don't come anywhere close to filling the space it controls, and creatures in combat are constantly moving. You can't reliably hide 3/4 of your body or more behind most creatures in combat.
The DM can make exceptions, but there should be circumstances that warrant it beyond the creature's size. For example, it's reasonable that a giant that was knocked prone and petrified would provide more than half cover, since it's both motionless and flat against the ground.
In general, you won't be able to fully protect other creatures from area effects, and attempting to predict an area effect with the Ready action is rarely worth it, as I mentioned earlier.
The PHB refers to the section discussing cover in the context of physical attacks, and the physical toughness of the material should be considered when determining cover. While a DM might rule that a stretched canvas provides cover against punches, the same canvas does not provide any cover against arrows dealing more than 1 hitpoint of damage. However, while by common sense, objects that would be pierced or destroyed partially during the duration of a damaging spell will not provide total cover, many spells do not deal any damage to overcome the physical barrier, regardless how filmsy the barrier might seem. Therefore, as long as the barrier is somewhat solid (including certain thickness of liquid), the target would get cover from the imaginary lines of effect.
So standing underwater in a ditch could protect a character from Fireball. If the damage of Fireball is not enough to incinerate the water during the duration of the spell, the character in the water would not need to make a DEX save. The lines of effect of Fireball is as flimsy as that of Silence, but the damaging effect of Fireball is destroying the covers during the duration of the effect, allowing the flimsy lines to extent further during the effect.
The same goes for dragon breath attacks. The effective reach of the range depends on the damage of the breath. So canvas could block paralyzing breath (which does not deal any damage) but not icy breath (where the canvas would shatter into pieces by cold damage and the breath will go through).
An obstacle in the context of the cover rules is implied to be something solid. Water doesn't provide cover, though underwater combat has its own rules that imposes disadvantage on many weapon attacks and grants creatures fully immersed in water resistance to fire damage.
Thank you for the link that in order to simplify the game, the game defines that a creature can only provide half cover.
I am okay with an interpretation that any cover needs to be "solid enough" to absorb at least a few hitpoints of piercing damage even if the spell itself is not dealing damage. So a sack can't block the lines, but many layers of them could. Tables would be solid enough to totally obstruct the lines.
But if the point of origin is irrelevant after a spell is cast, then:
1. If I cast Sleet Storm, in a square room on the floor, and the walls and ceiling are destroyed later, would the Sleet Storm remain a cube instead of extending to its full range as a cylinder? What happens when I cast Sleet Storm in front of a sailing ship? Does the Cylinder sweep through every cabin of the ship ignoring the cover that the roof would normally provide?
2. If I cast Antilife shell while standing at a corner, the shape of the effect would become wedge shaped. As I walk away from the corner, does the wedge shape maintains itself for my allies to walk within 5 feet of me? What happens when I turn. Does my wedge shaped shell turn with me? Can I cast Antilife Shell while standing in a hole to make an Antilife Shell umbrella coming from my fist only? Can I then position the Shell in any direction like a shield to protect people next to me against grapples?
Without any official reference, I think what would make sense is that the origin continues to affect the shape of the affected area during the duration. When the duration of the spell is instantaneous, it seems that the shape is defined at the moment it is cast. But when the duration is longer, it can be seen that the origin is continually defining the affected area. In the case of a ship going through a Sleet Storm, the roof and hull of the ship can still be providing total cover when the ship is going through. The storm does not automatically affect everything inside. In the case of the Antilife Shell, the Shell expands when the character walks away from the wall. The shape is not only continually centered, but continually redefined by its origin and relative position of any cover that may now block the lines.
I try to separate discussion about what the game mechanics allow and whether an the action is practical or worthwhile because it distracts from understanding the rules. We also can't really judge the worth of an action without knowing the objective of the encounter.
I see now that you can't ready to "Dash". But you can ready to move, which was what I meant. According to PHB, you can move, then take the Ready action to move again with your remaining speed during your reaction. So a character could move when the dragon moves to get out of range or behind a cover.
You could only use a readied action to move out of an AoE only if the DM let you use your readied movement after the AoE target was decided but before the attack took effect. I'm not entirely sure such a moment in time exists. I accept that the character might be able to see a Dragon is preparing to breathe fire, but not that they could predict exactly where that fire would go, until it was too late.
I feel like very few of the things you have described in your examples are actually governed by the rules as written. They all fall under DM judgement. If a situation came up in a game like any of these it would be ruled quickly and not up for a detailed debate on the semantics of whether cloth is sufficient cover from X effect. If you want to reliably avoid those effects then probably choose a solid wall to be safe.
I think there are benefits to decide those things beforehand so that the rulings are consistent. If a DM and the players know what tactics are possible, then the DM can design the encounter to allow the players to use tactics.
Example:
O O D O O <- D = Dragon. X = Cover, M = Mage. T = Tank/Spotter O T O O O Initially, the Mage has cover against the dragon. O OXXO O The Mage has no reason to move. O OXXO O Magereadies an action to move if the dragon moves to get around the cover. O O M O O
O O O O O O T O O O<- The dragon moves during its turn. O OXXD O The Tank yells that the Dragon is flying to the right side. O OXXO O The Mage knows that the Dragon does not have enough movement remaining to get to the left side. O O M O O This is the trigger for the Mage to know to move left, right, or not at all.
O O O O O O T O O O O OXXO O O MXXDO<- The Mage moves to the left side of the cover as the Dragon continues trying to get around. O O O O O Result: The Mage has moved away from the Dragon's attack range without knowing whether the Dragon would in fact attack.
That's all well and good but like I said the Ready action is usually worthless defensively. You lose your action to try to protect yourself against a single occurrence of some circumstance that may not happen, and if it does you lose your reaction as well. The Dodge action and spells like Shield, Absorb Elements, Blur, Mirror Image, or Haste are usually a better way to defend yourself.
The Ready action makes more sense when you want to attack after an enemy moves into a certain position (or comes out of hiding) or an ally imposes some condition on them (e.g. Guiding Bolt).
Tactics are situational. In the example above, none of the other options (Dodge, Shield, Absorb Elements, Blur, Mirror Image, Haste) could have prevented the Dragon from attacking the Mage, or do anything at all against a breath attack.
Suppose the Mage had cast Haste on the Tank and this is the beginning of battle. If the Mage gets hit the first round and loses concentration (or gets KO'ed), the Tank would lose a turn because haste would fade. If the Mage was standing anywhere else on the map above and did not ready to move, the Mage would have been hit by breath attack for sure.
Other factors that could make such tactic favorable include: 1. The Mage has no spell slot left. 2. The Mage has is at 1 hit point left. 3. The Mage is concentrating on an important spell. 4. The Dragon is only interested in attacking the Mage. 5. The Mage is casting a spell that takes 1 minute to cast. The Dragon breaches the chamber when there are only two turns left for the spell to complete. Edit: A character casting such a spell must spend the action to cast the spell. So the character can't take the Ready action.
There could be merit in an argument that a certain action will NEVER be better than another option that is ALWAYS available. But I think we are in agreement that this action (Ready to move) can be the BEST option in SOME situation, therefore, it should not be dismissed as a valid tactic.
Now we know that such tactic exists, a DM can consider this tactic to vary the encounters.
How Ready to Move may be included in an adventure:
The situation:
Two heroes (a tank and a mage) ventured into a temple ruin to retrieve a relic stolen centuries ago. At the main hall of the temple, they fought an exhaustive battle against but endless waves of stone golems before they figured out the trick to release the relic from the altar. As soon as the relic was removed, the golems turned into stone powder, but combat was not over. A stone statue of a 10 feet tall undead fire dragon near the entrance came into life as a fire dragon, blocking the path to the only entrance to the main hall. By now the heroes have low hit point and have no spell slots left. The mage has a special scroll to cast teleport circle, given by the new mayor quest giver for their quick return to the starting town. Once activated, the scroll casts itself but it takes 1 minute.
Dungeon layout:
On the back wall of the main hall are two small chambers on opposite sides. The chamber to the left is blocked by a partially collapsed thick wall with an opening large enough for them but too small for the dragon. The chamber to the right is undamaged and has a large pillar in the middle of its low 10 feet tall ceiling. The chamber appears to be a dead end. The architecture of the temple is symmetrical so far.
Players' assessment and plan:
The heroes assess that they cannot defeat the dragon, and should not remain in the main hall, which is large enough for the dragon to fly. They also assess that even if they could lure the dragon to move behind the pillar in the chamber on the right side, they don't have a mean to trap the dragon there or to outrun the dragon across the main hall to escape through the entrance. Based on symmetry, the heroes infer that that the chamber to the left is a similar dead end with a pillar in the middle. The collapsed wall may stall the dragon long enough for them to cast teleport circle to escape.
The heroes then dash to the left chamber and squeeze through the opening. The chamber indeed has the same layout: a dead end with a large pillar in the middle. The mage activated the teleport scroll behind the pillar. The dragon flies to the opening and makes a breath attack into the opening, but the pillar provides cover for both heroes from the blackish flame. Seeing that the breath attack was useless, the dragon starts attacking the collapsed wall. If the wall is destroyed, the dragon can move in to attack anywhere in that dead end.
Can the heroes escape through the circle before the wall breaks?
DM setting the stage for the final encounter:
"No," the DM said.
DM explains that the dragon will breach the wall one round before the circle completes. When the wall crumbles, the heroes will take their turns first, and the hero closer to the opening will have top initiative. Regardless of the initiative order, the teleportation circle will complete at the start of Mage's second turn after the wall is destroyed. If the circle completes, the portal will remain open until the end of Mage's third turn. From the battle map, it is clear that if the wall is breached, the dragon can move to and attack any space within the chamber on its turn. DM places where the dragon will be when the wall is breached, and asks the players to place their characters on the battle map to indicate where they will be at that moment.
The players position them as below. Mage is carrying the relic in one hand. Tank is wielding a sword and a shield.
XXXDXX D=Dragon, T=Tank, M=Mage. Mage is casting a teleportation circle. X T O OX Scale: Each O is about 10 sqft. XOXXOX Heroes can move 30 ft/turn. Dragon can moves 40 ft/turn. The space is too cramped to use wings. XOXXOX (Once the wall is breached, there is no space that is safe from the dragon.) XO(M)OX Pink O= Trigger locations conceived by the players. When dragon moves to either spot, Tank knows where Mage should take cover... XXXXXX ... the dragon will not have enough movement left to change course and attack the other side.
As positioned, the initiative order is Tank, Mage, then Dragon.
The final encounter:
On Tank's turn, Tank does not attack the dragon, but readies to move to retreat if the dragon moves to the left in order to remain outside the melee reach of Dragon. Tank intents to confirm which way the dragon will move. He has his shield ready to block debris. On Mage's turn, Mage stays behind the pillar with the scroll and the relic, away from the debris coming from both sides of the pillar. Mage readies to move to take new cover on trigger of Tank's signal. On the start of Dragon's turn, the wall has crumbled enough and the dragon squeezes through. Its shoulders rupture the collapsed wall, sending many debris forward. Tank needs to make a DEX save and succeeds. As the collapsing wall leaves a trail of debris behind the dragon, Tank sees and yells that the dragon is going to the right side. On that trigger, Mage uses her reaction to move to the left side of the pillar. Although the dragon knows exactly where everything is, it has spent all of its movement. Neither hero is within its attack range. Unknown to the heroes, the dragon then readies its breath attack, waiting for the hero carrying the relic to emerge. Since the tank's trigger did not occur, he uses his reaction to move next to the mage along the left side. First round ends like this:
XXX@XX @ = Difficult terrain. (C) = incomplete teleporation circle X@@@X (If the tank did not stay to confirm which way the dragon went, Mage may guess XTXX@X wrong from the noise and move to the wrong side. Mage may also move too XMXXDX early or too late. If Mage moves too early, Dragon may change course, and Mage XO(C)OX will find the dragon face-to-face. Reacting too late, Mage will be caught in the XXXXXX range or breath attack.)
On the second round, at Tank's turn, Tank moves to the edge of the pillar and readies to step into the circle on the trigger that the circle is complete. At the start of Mage's turn, the circle is complete and the two rush to the circle together. As they come within the dragon's attack range, the dragon uses its reaction to make a breath attack. The DM asks both heroes to make a DEX save with disadvantage. Both heroes roll low.
Concluding the adventure:
The DM narrates that as the heroes rush to the teleportation circle, the dragon releases its readied fire breath attack at point blank. All the heroes can see is a bright light surrounding them, completely engulfed by something that is not hot, but rather refreshing. When they come to their senses, they see that they are completely soaked by water. The heroes are now sitting in the water pool of the new water fountain, in the center of the town plaza of the starting town.
The teleportation was instantaneous. Although the dragon unleashed the breath attack as soon as the two lose cover, the attack missed both heroes by a split second. The two heroes safely teleported back to town, with the relic, but both heroes fell prone in the water fountain because they did not know that the teleport would exit there. They are now sitting in the shallow water pool, soaked wet by the water pouring over their heads from the water fountain.
As the water-soaked heroes take comical relief about the absurdity of placing a water fountain in the middle of a teleportation circle, the DM explains that most of the townsfolk do not know about magic and would not have known that there was a teleportation circle there. Then the DM asks the players to make another DEX save with disadvantage.
DM narrates that as the heroes are sitting in the water, they notice that the dragon is coming down from the portal above them. The heroes roll off to dodge the falling dragon. The dragon lands and crushes the water fountain in a loud explosion. A large chunk of the water fountain breaks to fall onto the mage, but the tank's aura deflected it, making it land just next to the mage. DM rolls a d6. The townsfolk screams as the dragon spreads its wings and a fiery glow rises up to the dragon's throat. It is still the dragon's turn and its breath attack has recharged. This time, there is no cover and neither hero has readied any reaction. The dragon aims squarely and intensely at both of the prone heroes.
As the dragon is about to attack, the relic resonates. A hidden circle of sigil lights up in the water pool. The water levitates and surrounds the dragon in a holy light. Within a few seconds the dragon's fiery glow discharges. The dragon turns into a crystal statue. Then the light of the both circles of sigil fades away.
The quest giver mayor emerges from the confused and frightened crowd, displeased at the destruction of the new water fountain. He says, "The contract speaks of no statue!"
The heroes say, "Precisely!"
After the heroes finish their businesses with the mayor, the heroes learn from an old townsfolk that the dragon was a friend of the town centuries ago. When the relic was stolen, the dragon went off searching but never returned. The people placed the sigil as a prayer for the dragon's safe return. Recalling the mechanism to remove the relic, the heroes understand how the powerful dragon would be trapped in the main hall and be defeated by the golems eventually.
As they pay a final tribute to the dragon before leaving town, the heroes see that, although frozen in an attack pose, the dragon statue was staring down peacefully, at the ground where a new sigil reads, "Welcome home, my dear friend!"
HI!
I am trying to understand how cover may or may not help against AOE spells. From what I understand from the rules, total cover affects the line of effect check to determines whether the target is affected by the spell at all. Then, if the spell requires a DEX save, depending on which direction(s) the effect reaches the target, the target gets the minimum DEX bonus from the maximum DEX bonus of each direction. When the line of effect can bend infinitely many times, unless the target has Total Cover within range from all sides, the line of effect could eventually reach the target from an opening.
Then:
1. A cover that wraps around the entire body could provide cover against spells that have a bending line of effect.
Example: A familiar that is in a pocket or within the armor of another character does not need to roll for DEX save against Fireball when it is totally covered from all sides.
2. Walls and Doors provide total cover for spells originating from the other side.
Example: A caster can't cast Antilife Shell from the other side of the door to prevent other characters from reaching for the doorknob because the door blocks the magic. A character in full plate armor cannot enter the Antilife Shell even with an opague visor because typically the armors and shields of a character are parts of the character "creature". However, a big sack can be dropped onto and enclose the caster, then the Antilife Shell can only extend to the interior wall of the bag, and other characters can all approach and tickle the caster through the bag. If the caster casts Antimagic Field, the Field would reach up to the bag including the bag. The bag itself can no longer be affected by telekinesis because the bag cannot provide Total Cover for itself. To telekinesis the bagged caster, another bag would need to enclose the bagged mage.
3. Obstructing the origin of an AOE effectively cancels the effect of the AOE (unless the spell specifically says otherwise).
Example: Sleet Storm has an origin at the bottom face of its cylinder. After the spell is cast, if someone places a shield or sits at the origin, the floor remains icy but the heavily obscuring storm is gone everywhere else besides the origin. After Silence is cast, a character could walk to the center, thus its own body is blocking the origin of the spell. A creature cannot provide Total Cover for itself, so the character obstructing the origin of the Silence spell is silenced, but other characters in the sphere become unaffected.
4. A character can ready Dash action to dash out of AOE range, to dash behind cover, or to dash to provide cover.
Example: Dashing from physical attack means that as the character sees the dragon flying to them, they dash away hoping to continually keep away from the dragon's physical reach. In these cases the trigger of the dash action is the movement of the dragon. The character would need to know where the dragon is. The trigger cannot be "when the dragon attacks", because that will be too late.
5. Total Cover can block dragon breath attacks.
Example: Depending on positioning and size, Floating Disk, object under Telekinesis, and creatures can all provide Total Cover. PHB gives an example that a creature can provide half cover. That is not a rule that a creature can ONLY provide half cover. A creature can also provide Total Cover depends on the size and positioning. A character could ready a dash action to continually position itself behind Total Cover or to provide Total Cover to another character, when the dragon moves. The trigger of the dash action needs to be the dragon's movement. When the dragon attacks, it is too late for the reaction to dash and provide cover. If a character is providing Total Cover to another character, regardless whether the Cover succeed or fail the CON save, the covered character is not affected by the breath attack.
I think you may be overthinking some of those, or you may be looking at the rules a little too closely (to the extend of considering semantics). Cover rules only mention examples because it's impossible to list every situation and the cover bonus it would include, so they're just guidelines so that the DM can make appropriate calls.
I like the breakdown, though, so I thought I'd give you my personal opinion (and potential handling) if such situations (and/or attempts) came up to one of my games, if that helps. :)
1. Sure, I don't think I'd kill the healer's celestial hamster companion if she always keeps it in a pocket or something. Rather than go with a bonus to save, I'd give it total cover unless it was doing something specific when catastrophe struck. I'd possibly consider the circumstances more carefully if it was a character that was miniaturized somehow and hid in a pocket, but I doubt I would, unless it was an attempt to exploit it.
2. Antilife Shell versus door? Interesting, and I'd likely err towards the players' ideas the first time it came up (and keep consistent with that handling thereafter). Sack providing total cover is too far, though - a sack isn't an obstacle any more than a character's clothes are, and it's not going to stop an Antilife Shell any more than it's going to stop a Silence spell or a Fireball. Trying to cover yourself up with a cloak to stop the damage from a cave-in is an inside joke to my group, but it didn't help the first time it was tried either. :p
3. Sure, but providing total cover isn't as simple as you seem to suggest (or infer by the rules), and is much less so when trying to figure out a point of origin of an effect (such as the center of a Silence spell. It's one thing to consider situations out of combat, where you can put your hand in front of a magical line of green light to stop it from shining on that emerald, but quite another to try to fully block an area of effect with your body in combat. You do not fill your space, so you technically never provide total cover in combat, other than the DM judging for dramatic appropriateness that jumping on the glowing orb of a Delayed Blast Fireball will stop it from affecting your companions (and will likely obliterate you).
4. Yes! If you can figure out a good trigger (perhaps the dragon scrounges up his nose every time he's about to breathe fire?), and the opponent isn't smart enough to notice what you're doing and bluff you (some dragons can be stupid!), this is a viable tactic. You do usually lose your action (and your reaction) to fully utilize such a defensive Ready, though, so it's hardly an efficient course of action - unless you have means to help a lot with Bonus Actions or similar, such as Healing Word or a Thief's Cunning Action. Or a Fighter's Action Surge - that's always good for a last ditch attempt to do something stylish. :p
5. Sure, but see 3. A Wall of Force (or other solid walls) is ideal for such things. A cloth bag isn't. :p
Solid armor could arguably provide cover to a small familiar, but you'd be hard-pressed to find a gap in armor where a familiar could fit and also not be crushed to death. Clothing isn't a solid obstacle and provides no cover. If you want to keep your familiar safe, dismiss it.
A bag isn't a solid obstacle and isn't damage-proof enough to provide total cover.
Nope. The effect for an area spell radiates out from the point of origin at the moment it was cast. Once the spell effect has been produced, the point of origin becomes irrelevant.
Also keep in mind the point of origin for a cylinder can be at the height of the spell, and the point of origin for a cube can be anywhere on its surface.
You could ready an action to move out of the way if an area effect is preceded by a perceptible circumstance, but this is rarely practical; if the area effect doesn't happen, you've wasted your action. Taking the Dodge is usually a better idea, and it also protects you against attacks.
Readying the Dash action arguably doesn't do anything, since it merely adds movement but you can't move when it's not your turn.
Tenser's Floating Disk generally won't be of much help as total cover unless you can predict that something is going to fall on you.
A creature only provides half cover, regardless of size. Unless it's a gelatinous cube, most creatures don't come anywhere close to filling the space it controls, and creatures in combat are constantly moving. You can't reliably hide 3/4 of your body or more behind most creatures in combat.
The DM can make exceptions, but there should be circumstances that warrant it beyond the creature's size. For example, it's reasonable that a giant that was knocked prone and petrified would provide more than half cover, since it's both motionless and flat against the ground.
In general, you won't be able to fully protect other creatures from area effects, and attempting to predict an area effect with the Ready action is rarely worth it, as I mentioned earlier.
The Forum Infestation (TM)
The penetration power of lines of effect
The PHB refers to the section discussing cover in the context of physical attacks, and the physical toughness of the material should be considered when determining cover. While a DM might rule that a stretched canvas provides cover against punches, the same canvas does not provide any cover against arrows dealing more than 1 hitpoint of damage. However, while by common sense, objects that would be pierced or destroyed partially during the duration of a damaging spell will not provide total cover, many spells do not deal any damage to overcome the physical barrier, regardless how filmsy the barrier might seem. Therefore, as long as the barrier is somewhat solid (including certain thickness of liquid), the target would get cover from the imaginary lines of effect.
So standing underwater in a ditch could protect a character from Fireball. If the damage of Fireball is not enough to incinerate the water during the duration of the spell, the character in the water would not need to make a DEX save. The lines of effect of Fireball is as flimsy as that of Silence, but the damaging effect of Fireball is destroying the covers during the duration of the effect, allowing the flimsy lines to extent further during the effect.
The same goes for dragon breath attacks. The effective reach of the range depends on the damage of the breath. So canvas could block paralyzing breath (which does not deal any damage) but not icy breath (where the canvas would shatter into pieces by cold damage and the breath will go through).
An obstacle in the context of the cover rules is implied to be something solid. Water doesn't provide cover, though underwater combat has its own rules that imposes disadvantage on many weapon attacks and grants creatures fully immersed in water resistance to fire damage.
The Forum Infestation (TM)
Thank you for the link that in order to simplify the game, the game defines that a creature can only provide half cover.
I am okay with an interpretation that any cover needs to be "solid enough" to absorb at least a few hitpoints of piercing damage even if the spell itself is not dealing damage. So a sack can't block the lines, but many layers of them could. Tables would be solid enough to totally obstruct the lines.
But if the point of origin is irrelevant after a spell is cast, then:
1. If I cast Sleet Storm, in a square room on the floor, and the walls and ceiling are destroyed later, would the Sleet Storm remain a cube instead of extending to its full range as a cylinder? What happens when I cast Sleet Storm in front of a sailing ship? Does the Cylinder sweep through every cabin of the ship ignoring the cover that the roof would normally provide?
2. If I cast Antilife shell while standing at a corner, the shape of the effect would become wedge shaped. As I walk away from the corner, does the wedge shape maintains itself for my allies to walk within 5 feet of me? What happens when I turn. Does my wedge shaped shell turn with me? Can I cast Antilife Shell while standing in a hole to make an Antilife Shell umbrella coming from my fist only? Can I then position the Shell in any direction like a shield to protect people next to me against grapples?
Without any official reference, I think what would make sense is that the origin continues to affect the shape of the affected area during the duration. When the duration of the spell is instantaneous, it seems that the shape is defined at the moment it is cast. But when the duration is longer, it can be seen that the origin is continually defining the affected area. In the case of a ship going through a Sleet Storm, the roof and hull of the ship can still be providing total cover when the ship is going through. The storm does not automatically affect everything inside. In the case of the Antilife Shell, the Shell expands when the character walks away from the wall. The shape is not only continually centered, but continually redefined by its origin and relative position of any cover that may now block the lines.
I try to separate discussion about what the game mechanics allow and whether an the action is practical or worthwhile because it distracts from understanding the rules. We also can't really judge the worth of an action without knowing the objective of the encounter.
Ready to move
I see now that you can't ready to "Dash". But you can ready to move, which was what I meant. According to PHB, you can move, then take the Ready action to move again with your remaining speed during your reaction. So a character could move when the dragon moves to get out of range or behind a cover.
You could only use a readied action to move out of an AoE only if the DM let you use your readied movement after the AoE target was decided but before the attack took effect. I'm not entirely sure such a moment in time exists. I accept that the character might be able to see a Dragon is preparing to breathe fire, but not that they could predict exactly where that fire would go, until it was too late.
I feel like very few of the things you have described in your examples are actually governed by the rules as written. They all fall under DM judgement. If a situation came up in a game like any of these it would be ruled quickly and not up for a detailed debate on the semantics of whether cloth is sufficient cover from X effect. If you want to reliably avoid those effects then probably choose a solid wall to be safe.
I think there are benefits to decide those things beforehand so that the rulings are consistent. If a DM and the players know what tactics are possible, then the DM can design the encounter to allow the players to use tactics.
Example:
O O D O O <- D = Dragon. X = Cover, M = Mage. T = Tank/Spotter
O T O O O Initially, the Mage has cover against the dragon.
O OXXO O The Mage has no reason to move.
O OXXO O Magereadies an action to move if the dragon moves to get around the cover.
O O M O O
O O O O O
O T O O O <- The dragon moves during its turn.
O OXXD O The Tank yells that the Dragon is flying to the right side.
O OXXO O The Mage knows that the Dragon does not have enough movement remaining to get to the left side.
O O M O O This is the trigger for the Mage to know to move left, right, or not at all.
O O O O O
O T O O O
O OXXO O
O MXXDO <- The Mage moves to the left side of the cover as the Dragon continues trying to get around.
O O O O O Result: The Mage has moved away from the Dragon's attack range without knowing whether the Dragon would in fact attack.
That's all well and good but like I said the Ready action is usually worthless defensively. You lose your action to try to protect yourself against a single occurrence of some circumstance that may not happen, and if it does you lose your reaction as well. The Dodge action and spells like Shield, Absorb Elements, Blur, Mirror Image, or Haste are usually a better way to defend yourself.
The Ready action makes more sense when you want to attack after an enemy moves into a certain position (or comes out of hiding) or an ally imposes some condition on them (e.g. Guiding Bolt).
The Forum Infestation (TM)
Tactics are situational. In the example above, none of the other options (Dodge, Shield, Absorb Elements, Blur, Mirror Image, Haste) could have prevented the Dragon from attacking the Mage, or do anything at all against a breath attack.
Suppose the Mage had cast Haste on the Tank and this is the beginning of battle. If the Mage gets hit the first round and loses concentration (or gets KO'ed), the Tank would lose a turn because haste would fade. If the Mage was standing anywhere else on the map above and did not ready to move, the Mage would have been hit by breath attack for sure.
Other factors that could make such tactic favorable include:
1. The Mage has no spell slot left.
2. The Mage has is at 1 hit point left.
3. The Mage is concentrating on an important spell.
4. The Dragon is only interested in attacking the Mage.
5.
The Mage is casting a spell that takes 1 minute to cast. The Dragon breaches the chamber when there are only two turns left for the spell to complete.Edit: A character casting such a spell must spend the action to cast the spell. So the character can't take the Ready action.There could be merit in an argument that a certain action will NEVER be better than another option that is ALWAYS available. But I think we are in agreement that this action (Ready to move) can be the BEST option in SOME situation, therefore, it should not be dismissed as a valid tactic.
Now we know that such tactic exists, a DM can consider this tactic to vary the encounters.
How Ready to Move may be included in an adventure:
The situation:
Two heroes (a tank and a mage) ventured into a temple ruin to retrieve a relic stolen centuries ago. At the main hall of the temple, they fought an exhaustive battle against but endless waves of stone golems before they figured out the trick to release the relic from the altar. As soon as the relic was removed, the golems turned into stone powder, but combat was not over. A stone statue of a 10 feet tall undead fire dragon near the entrance came into life as a fire dragon, blocking the path to the only entrance to the main hall. By now the heroes have low hit point and have no spell slots left. The mage has a special scroll to cast teleport circle, given by the new mayor quest giver for their quick return to the starting town. Once activated, the scroll casts itself but it takes 1 minute.
Dungeon layout:
On the back wall of the main hall are two small chambers on opposite sides. The chamber to the left is blocked by a partially collapsed thick wall with an opening large enough for them but too small for the dragon. The chamber to the right is undamaged and has a large pillar in the middle of its low 10 feet tall ceiling. The chamber appears to be a dead end. The architecture of the temple is symmetrical so far.
Players' assessment and plan:
The heroes assess that they cannot defeat the dragon, and should not remain in the main hall, which is large enough for the dragon to fly. They also assess that even if they could lure the dragon to move behind the pillar in the chamber on the right side, they don't have a mean to trap the dragon there or to outrun the dragon across the main hall to escape through the entrance. Based on symmetry, the heroes infer that that the chamber to the left is a similar dead end with a pillar in the middle. The collapsed wall may stall the dragon long enough for them to cast teleport circle to escape.
The heroes then dash to the left chamber and squeeze through the opening. The chamber indeed has the same layout: a dead end with a large pillar in the middle. The mage activated the teleport scroll behind the pillar. The dragon flies to the opening and makes a breath attack into the opening, but the pillar provides cover for both heroes from the blackish flame. Seeing that the breath attack was useless, the dragon starts attacking the collapsed wall. If the wall is destroyed, the dragon can move in to attack anywhere in that dead end.
Can the heroes escape through the circle before the wall breaks?
DM setting the stage for the final encounter:
"No," the DM said.
DM explains that the dragon will breach the wall one round before the circle completes. When the wall crumbles, the heroes will take their turns first, and the hero closer to the opening will have top initiative. Regardless of the initiative order, the teleportation circle will complete at the start of Mage's second turn after the wall is destroyed. If the circle completes, the portal will remain open until the end of Mage's third turn. From the battle map, it is clear that if the wall is breached, the dragon can move to and attack any space within the chamber on its turn. DM places where the dragon will be when the wall is breached, and asks the players to place their characters on the battle map to indicate where they will be at that moment.
The players position them as below. Mage is carrying the relic in one hand. Tank is wielding a sword and a shield.
XXXDXX D=Dragon, T=Tank, M=Mage. Mage is casting a teleportation circle.
X T O OX Scale: Each O is about 10 sqft.
XOXXOX Heroes can move 30 ft/turn. Dragon can moves 40 ft/turn. The space is too cramped to use wings.
XOXXOX (Once the wall is breached, there is no space that is safe from the dragon.)
XO(M)OX Pink O = Trigger locations conceived by the players. When dragon moves to either spot, Tank knows where Mage should take cover...
XXXXXX ... the dragon will not have enough movement left to change course and attack the other side.
As positioned, the initiative order is Tank, Mage, then Dragon.
The final encounter:
On Tank's turn, Tank does not attack the dragon, but readies to move to retreat if the dragon moves to the left in order to remain outside the melee reach of Dragon. Tank intents to confirm which way the dragon will move. He has his shield ready to block debris. On Mage's turn, Mage stays behind the pillar with the scroll and the relic, away from the debris coming from both sides of the pillar. Mage readies to move to take new cover on trigger of Tank's signal. On the start of Dragon's turn, the wall has crumbled enough and the dragon squeezes through. Its shoulders rupture the collapsed wall, sending many debris forward. Tank needs to make a DEX save and succeeds. As the collapsing wall leaves a trail of debris behind the dragon, Tank sees and yells that the dragon is going to the right side. On that trigger, Mage uses her reaction to move to the left side of the pillar. Although the dragon knows exactly where everything is, it has spent all of its movement. Neither hero is within its attack range. Unknown to the heroes, the dragon then readies its breath attack, waiting for the hero carrying the relic to emerge. Since the tank's trigger did not occur, he uses his reaction to move next to the mage along the left side. First round ends like this:
XXX@XX @ = Difficult terrain. (C) = incomplete teleporation circle
X@@@X (If the tank did not stay to confirm which way the dragon went, Mage may guess
XTXX@X wrong from the noise and move to the wrong side. Mage may also move too
XMXXDX early or too late. If Mage moves too early, Dragon may change course, and Mage
XO(C)OX will find the dragon face-to-face. Reacting too late, Mage will be caught in the
XXXXXX range or breath attack.)
On the second round, at Tank's turn, Tank moves to the edge of the pillar and readies to step into the circle on the trigger that the circle is complete. At the start of Mage's turn, the circle is complete and the two rush to the circle together. As they come within the dragon's attack range, the dragon uses its reaction to make a breath attack. The DM asks both heroes to make a DEX save with disadvantage. Both heroes roll low.
Concluding the adventure:
The DM narrates that as the heroes rush to the teleportation circle, the dragon releases its readied fire breath attack at point blank. All the heroes can see is a bright light surrounding them, completely engulfed by something that is not hot, but rather refreshing. When they come to their senses, they see that they are completely soaked by water. The heroes are now sitting in the water pool of the new water fountain, in the center of the town plaza of the starting town.
The teleportation was instantaneous. Although the dragon unleashed the breath attack as soon as the two lose cover, the attack missed both heroes by a split second. The two heroes safely teleported back to town, with the relic, but both heroes fell prone in the water fountain because they did not know that the teleport would exit there. They are now sitting in the shallow water pool, soaked wet by the water pouring over their heads from the water fountain.
As the water-soaked heroes take comical relief about the absurdity of placing a water fountain in the middle of a teleportation circle, the DM explains that most of the townsfolk do not know about magic and would not have known that there was a teleportation circle there. Then the DM asks the players to make another DEX save with disadvantage.
DM narrates that as the heroes are sitting in the water, they notice that the dragon is coming down from the portal above them. The heroes roll off to dodge the falling dragon. The dragon lands and crushes the water fountain in a loud explosion. A large chunk of the water fountain breaks to fall onto the mage, but the tank's aura deflected it, making it land just next to the mage. DM rolls a d6. The townsfolk screams as the dragon spreads its wings and a fiery glow rises up to the dragon's throat. It is still the dragon's turn and its breath attack has recharged. This time, there is no cover and neither hero has readied any reaction. The dragon aims squarely and intensely at both of the prone heroes.
As the dragon is about to attack, the relic resonates. A hidden circle of sigil lights up in the water pool. The water levitates and surrounds the dragon in a holy light. Within a few seconds the dragon's fiery glow discharges. The dragon turns into a crystal statue. Then the light of the both circles of sigil fades away.
The quest giver mayor emerges from the confused and frightened crowd, displeased at the destruction of the new water fountain. He says, "The contract speaks of no statue!"
The heroes say, "Precisely!"
After the heroes finish their businesses with the mayor, the heroes learn from an old townsfolk that the dragon was a friend of the town centuries ago. When the relic was stolen, the dragon went off searching but never returned. The people placed the sigil as a prayer for the dragon's safe return. Recalling the mechanism to remove the relic, the heroes understand how the powerful dragon would be trapped in the main hall and be defeated by the golems eventually.
As they pay a final tribute to the dragon before leaving town, the heroes see that, although frozen in an attack pose, the dragon statue was staring down peacefully, at the ground where a new sigil reads, "Welcome home, my dear friend!"