I thought of a few things that could come up in combat where rules start to overlap and be messy, so where would you put rulings for these things. Feel free to vote and comment or just one or the other or just read to see what people say. The polls are as follows
Mobile "Don't Provoke oppurtunity attack if you attack a creature with melee" vs. Sentinel "Creature provoke oppurtunity attack, even if they Disengage, when they leave your reach."
Level 5 Champion Fighter Hits with a Critical on the first attack and misses on the second vs. Level 5 Thief Rogue uses uncanny dodge to take away half the damage.
Shield spell react automatically to enemy attacks you taking away reaction vs. Shield spell reacting only when you would have taken damage (can still use reactions if they miss).
Level 1 Variant Human Fighter with Magic Initiate Feat vs. Level 1 Variant Human Wizard with Weapon Master Feat.
Mobile "wins". Sentinel prevents the "Disengage" action from doing anything, but mobile is a different thing, Sentinel has no interaction with the mobile feat.
I don't even get the question you have champion fighter and rogue. The champion fighter gets a crit, so they roll double dice to figure out damage. The rogue can use their reaction uncanny dodge to take half of that damage instead of the full amount, if they can see the attacker. How does the second attack factor into it at all? The rogue has to decide whether to use their reaction for uncanny dodge after the first attack, before they know whether the second one hit or not. I don't even understand what you're asking about.
Shield specifies that you can cast it when you are hit by an attack or Magic Missile. It can only be used if you've been hit by the attack or targeted by the MM. If you haven't been hit (such as because the enemy did not roll well enough to hit your normal AC) then you can't cast Shield.
What are you even asking about between fighter with magic initiate and wizard with weapon master? Which one we "like" more?
To the last poll: a fighter with Magic Initiate is still a fighter, but he can pick up several helpful utility cantrips and long lasting spells like Goodberry that improve his overall performance.
A wizard who can use a sword is still a wizard who has no business in melee. He still only gets one attack that won't hit due to his poor physical ability scores. The notable exception are Bladesingers. But they are a) not Variant Humans and b) get weapon proficiency at level 2 anyways, in addition to their elven weapon proficiency.
So for the fighter Magic Initiate is a valuable feat that improves his performance, while for the wizard Weapon Master is pretty much useless.
In regards to the point claiming that Uncanny Dodge working on a crit would make crits useless against rogues, it makes me wonder how much PvP combat you're running? Dnd is not a very robust PvP game, so judging player abilities' effectiveness against each other is sort of a moot point. Most of these abilities were designed to work in tandem with other characters, not against them. Without the ability to Uncanny Dodge, rogues (with their limited defensive capabilities and small hit dice) would not be able to stay in fights alongside their more heavily armored party members, and with an entire class unable to compete with others, the game becomes imbalanced to the point where, why even bother picking rogue?
So yes, a rogue should always be able to use their reaction for Uncanny Dodge unless they're physically restrained from doing so. Not to allow it would be a bigger balance issue. A rogue in single combat against a fighter is still in a sub-optimal position, as you've pointed out, as the fighter still has 2 attacks to the rogue's 1, and the rogue can only Uncanny Dodge once per turn. The fighter in that scenario is just unlucky that their second attack missed, but you could also make the case that they're lucky that on the turn he only landed one attack, it was at least a crit, so the rogue still took more damage than they would have from a regular hit, UD or no. It's still a net positive for the fighter.
So on the second one with the rogue and fighter, maybe to avoid PvP we just say the monster attacks twice and crits on one of them. Since the monster is attacking the rogue twice and the rogue only has one reaction they have to decide whether or not to UD the first attack before knowing the result of the second? Sure other monsters could also attack the rogue when he uses his reaction but in regards to the same action taken by 1 monster since all those attacks are part of the one action the rogue has decide whether the first one is halfed before knowing the second? What if first attack dealt 3 damage and the other dealt 10? The rogue would take 11 damage in the end not knowing the second would be greater instead of 8?
The original question just had to do with whether UD halves a critical hit or not, but this provides a new question. Would a rogue react to half one attack that hits after the attack, or would they react to half only one of the attacks that hits after the action?
So on the second one with the rogue and fighter, maybe to avoid PvP we just say the monster attacks twice and crits on one of them. Since the monster is attacking the rogue twice and the rogue only has one reaction they have to decide whether or not to UD the first attack before knowing the result of the second?
Yes, of course. Because the monster can make different decisions based on whether the first attack kills the rogue or not (and whether the rogue used UD on the first attack or not.) Turns happen sequentially, monsters get to do things in order just like players do.
Sure other monsters could also attack the rogue when he uses his reaction but in regards to the same action taken by 1 monster since all those attacks are part of the one action the rogue has decide whether the first one is halfed before knowing the second?
The rogue doesn't even know whether there will *be* a second attack - it might depend on how much damage he takes!
What if first attack dealt 3 damage and the other dealt 10? The rogue would take 11 damage in the end not knowing the second would be greater instead of 8?
Yep. The sequence is:
1) The rogue is attacked and gets dealt some damage.
2) The rogue gets to choose whether to use their reaction to react to that damage in some way, if they can - UD for example.
3) Then the monster gets to *decide* what to do next. It might be attacking the rogue again, it might be attacking someone else, it might be doing something totally different. The first attack might have changed the situation.
The original question just had to do with whether UD halves a critical hit or not, but this provides a new question.
Yes, UD halves a critical hit.
Would a rogue react to half one attack that hits after the attack, or would they react to half only one of the attacks that hits after the action?
Uncanny dodge says " when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you." So it applies to one attack, and happens as soon as an attacker you can see hits you with the attack, before finding out what else the attacker is going to do on their turn.
So on the second one with the rogue and fighter, maybe to avoid PvP we just say the monster attacks twice and crits on one of them. Since the monster is attacking the rogue twice and the rogue only has one reaction they have to decide whether or not to UD the first attack before knowing the result of the second?
Yes, of course. Because the monster can make different decisions based on whether the first attack kills the rogue or not (and whether the rogue used UD on the first attack or not.) Turns happen sequentially, monsters get to do things in order just like players do.
Sure other monsters could also attack the rogue when he uses his reaction but in regards to the same action taken by 1 monster since all those attacks are part of the one action the rogue has decide whether the first one is halfed before knowing the second?
The rogue doesn't even know whether there will *be* a second attack - it might depend on how much damage he takes!
What if first attack dealt 3 damage and the other dealt 10? The rogue would take 11 damage in the end not knowing the second would be greater instead of 8?
Yep. The sequence is:
1) The rogue is attacked and gets dealt some damage.
2) The rogue gets to choose whether to use their reaction to react to that damage in some way, if they can - UD for example.
3) Then the monster gets to *decide* what to do next. It might be attacking the rogue again, it might be attacking someone else, it might be doing something totally different. The first attack might have changed the situation.
The original question just had to do with whether UD halves a critical hit or not, but this provides a new question.
Yes, UD halves a critical hit.
Would a rogue react to half one attack that hits after the attack, or would they react to half only one of the attacks that hits after the action?
Uncanny dodge says " when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you." So it applies to one attack, and happens as soon as an attacker you can see hits you with the attack, before finding out what else the attacker is going to do on their turn.
"Then the monster gets to *decide* what to do next. It might be attacking the rogue again, it might be attacking someone else, it might be doing something totally different. The first attack might have changed the situation."
So a creature with multiattack can use the same action to hit a different creature? This isn't assuming the creature hit by the initial attack dies; I mean yeah that would make sense to have the second attack carry on to another creature within 5 foot range if melee, 10 foot if reach, and so on, but supposing the creature didn't die to the first attack which is still part of the monsters action could they decide mid-action to switch over to another creatures. Could players do this as well where the fighter at level 5 uses multiattack and action surge in combat to attempt a hit against 4 different creatures and not two attacks against one as the first action and then two attacks on a second as part of their second action, being able to change targets because it's a new action and not a new attack?
If attacks are separated from actions and not part of an action then we say that the monster can decide whether to try and hit the rogue again after the rogue UD their first attack, but if the attacks cannot be separated from the action, like how a character can't use an action to drink half a potion to restore just half the effects and give it to their buddy to drink the other half and take half the effect, then the attack action likewise couldn't be halfed. Even in the case of downed enemy there is the idea of negative hit points where the first hit brings them down to zero or even passed zero and the second hit goes further into the negative, possibly perma-death that creature.
In the end UD still only halves one attack that hits, if the creature has action surge or a bonus action or even just other creatures in the round come to hit the rogue, they can't react any more. Now as the DM you would know which of your monsters will attack the rogue, what you don't know is if they will hit or not, no one can know that until the roll is made. Knowing a second monster will move in to attack the rogue where the first monster is but the rogue doesn't know this they just deal with the first monster who after being hit sneak attack is going to use its action to attack the rogue taking the multiattack action to deal 2 hits. Both times hits, and while the rogue doesn't know you have plans to send the second monster to attack as well only gets access to the information the dice gave to them, whether the hit succeeded or not. The rogue can't react before knowing the roll because it's only when a result is shared that they know if they got hit or not and only by being hit can they react. So rather than having the rogue say they want to uncanny dodge that action but wait to see which attacks will hit and which won't (or deciding if there is one of the two or three or however many multiattacks that action takes that they decide to half just one of them) it's only upon being first hit that they either use UD or they give it up.
See that's kind of the idea of this poll was to look at peoples thoughts and to break things down further because different people have different interpretations as how they see the rules whether it's through their experiences playing the game or running it or reading sources or wherever it might be. Some of the information isn't ironclad so it's important to half discussions outside of a game so that they don't get bogged down by all this jargon. In the end with rule 0 the DM interprets the rules however they want so it doesn't really matter what anyone on here says, but different perspectives can be shared and since no one is the DM in here no one is the arbiter of how things works....
Though I guess I would concede an argument if a tweet from Jeremy Crawford is shared to at least say "Here is what the devs say, this was there intention" which is fair, it says if you want to play the game your own way go ahead but if you want to play RAW and RAI then this is how it's done.
None of these situations are at all ambiguous in the rules.
1) Mobile > Sentinel
Mobile: "When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not."
Sentinel: "Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
Sentinel only mentions Disengage, not any other effects while mobile very explicitly states you don't provoke op attacks.
2) Thief can choose to use Uncanny Dodge on any hit. They do NOT know whether the next hit would be a crit or not. Each hit of an Attack action or Multiattack has to be resolved separately in order. The rules explicitly allow players with extra attack to move between attacks of the attack action and as a DM I would apply this to creatures with multiattack as well. Uncanny Dodge works as well on crits as anything else.
3) Shield spell explicitly states that it can be used when HIT. I'm not sure what you are even asking with this question unless it is about some sort of homebrew.
4) This question is also unclear as to what you are looking for ... MI fighter makes much more sense than a WM wizard if you are considering melee. Fighter has better armor, hit points, shield, second wind ... if melee is the goal the fighter is the best choice. However, you don't specify level or anything else. If you want a wizard with a bit of melee capability then a Bladesinger makes far more sense than taking the WM feat.
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Basically, none of the situations you have described are ambiguous or difficult to figure out what the rules say. There are far more contentious options out there :)
For example: What is total cover? Does a Wall of Force provide total cover preventing the casting of spells that only require you to see your target? Does a thin fragile window pane provide total cover? Does the thin window pane prevent the casting of spells or the firing of a crossbow bolt through the glass? This is an issue where you will get very divisive answers :)
So on the second one with the rogue and fighter, maybe to avoid PvP we just say the monster attacks twice and crits on one of them. Since the monster is attacking the rogue twice and the rogue only has one reaction they have to decide whether or not to UD the first attack before knowing the result of the second?
Yes, of course. Because the monster can make different decisions based on whether the first attack kills the rogue or not (and whether the rogue used UD on the first attack or not.) Turns happen sequentially, monsters get to do things in order just like players do.
Sure other monsters could also attack the rogue when he uses his reaction but in regards to the same action taken by 1 monster since all those attacks are part of the one action the rogue has decide whether the first one is halfed before knowing the second?
The rogue doesn't even know whether there will *be* a second attack - it might depend on how much damage he takes!
What if first attack dealt 3 damage and the other dealt 10? The rogue would take 11 damage in the end not knowing the second would be greater instead of 8?
Yep. The sequence is:
1) The rogue is attacked and gets dealt some damage.
2) The rogue gets to choose whether to use their reaction to react to that damage in some way, if they can - UD for example.
3) Then the monster gets to *decide* what to do next. It might be attacking the rogue again, it might be attacking someone else, it might be doing something totally different. The first attack might have changed the situation.
The original question just had to do with whether UD halves a critical hit or not, but this provides a new question.
Yes, UD halves a critical hit.
Would a rogue react to half one attack that hits after the attack, or would they react to half only one of the attacks that hits after the action?
Uncanny dodge says " when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you." So it applies to one attack, and happens as soon as an attacker you can see hits you with the attack, before finding out what else the attacker is going to do on their turn.
"Then the monster gets to *decide* what to do next. It might be attacking the rogue again, it might be attacking someone else, it might be doing something totally different. The first attack might have changed the situation."
So a creature with multiattack can use the same action to hit a different creature? This isn't assuming the creature hit by the initial attack dies; I mean yeah that would make sense to have the second attack carry on to another creature within 5 foot range if melee, 10 foot if reach, and so on, but supposing the creature didn't die to the first attack which is still part of the monsters action could they decide mid-action to switch over to another creatures. Could players do this as well where the fighter at level 5 uses multiattack and action surge in combat to attempt a hit against 4 different creatures and not two attacks against one as the first action and then two attacks on a second as part of their second action, being able to change targets because it's a new action and not a new attack?
Yes... that's how attacks work. In no stage of combat do you have to declare a single target for your turn.
Each attack a creature has is essentially approached as a unique instance. A fighter cornered by two gnolls might choose to try and focus one of them down with both attacks, or might choose to attack each one once. That's always they're choice and nothing ever prevents them from doing that.
Now, let's go back to your rogue. A monster with, say 3 attacks, can attack the rogue with all three (regardless of which ones hit), and let's say the first one is a crit that gets Uncanny Dodged-- the monster *can* choose to continue with more attacks on the rogue, it *can* choose to spread the rest of its attacks onto other party members, it *can* choose to give up attacking altogether and whine about getting UD'd. An attacker makes this choice *every time* they attack, and is more influenced by how smart or strategic the DM wants this monster to behave.
Maybe the rogue UD's a crit on the first attack from a Smart Monster who knows that they can't do that again, and decides to press their advantage by hitting them another two times.
Maybe the rogue UD's a Stupid Monster who thinks "aaagh! Puny creature hard to hit! I kill sumfing else!" and tries to hit something else with their subsequent attacks. Both are valid responses that are entirely up to how the DM is running the monster, and how difficult the DM wants the encounter to be.
All that, however, is irrelevant to your original question. There is zero ambiguity in the answer to your original question: a rogue can always spend their reaction to UD, even against crits, regardless of whatever else happens that turn. Wether they decide to wait and see if they're about to get hit harder (or at all) before using their reaction, or save it to use on a bigger attacker that turn, is entirely up to player discretion.
Of those options, the rules are completely clear on the first three (sentinel only negates disengage, uncanny dodge has no restriction on crits, the trigger for shield is being hit) and the fourth is a judgment call between two bad options (a fighter with magic initiate should take abilities that don't depend on their spellcasting stat, a wizard is not going to touch weapon attacks after level 4).
So on the second one with the rogue and fighter, maybe to avoid PvP we just say the monster attacks twice and crits on one of them. Since the monster is attacking the rogue twice and the rogue only has one reaction they have to decide whether or not to UD the first attack before knowing the result of the second? Sure other monsters could also attack the rogue when he uses his reaction but in regards to the same action taken by 1 monster since all those attacks are part of the one action the rogue has decide whether the first one is halfed before knowing the second? What if first attack dealt 3 damage and the other dealt 10? The rogue would take 11 damage in the end not knowing the second would be greater instead of 8?
The original question just had to do with whether UD halves a critical hit or not, but this provides a new question. Would a rogue react to half one attack that hits after the attack, or would they react to half only one of the attacks that hits after the action?
My answer to the original question was the rogue could half the damage regardless of whether it was a crit or not. My answer to this new question would be that the rogue would have to choose to uncanny dodge the first attack before seeing the results of the second. I've yet to play at a table with a rogue high enough to have uncanny dodge, so I'm not sure how necessary using it on a 3 damage would be, but I would assume that it would only stress me if the attack would knock me unconcious, particularly since I'd only be reducing the damage by one. I'd have to know what my health looked like and more of the situation to know whether I'd try to half the 10. Considering that 2 hits for 13 damage is probably not a good sign against a rogue, I'd probably opt to half the 10.
In regards to the point claiming that Uncanny Dodge working on a crit would make crits useless against rogues, it makes me wonder how much PvP combat you're running? Dnd is not a very robust PvP game, so judging player abilities' effectiveness against each other is sort of a moot point. Most of these abilities were designed to work in tandem with other characters, not against them. Without the ability to Uncanny Dodge, rogues (with their limited defensive capabilities and small hit dice) would not be able to stay in fights alongside their more heavily armored party members, and with an entire class unable to compete with others, the game becomes imbalanced to the point where, why even bother picking rogue?
So yes, a rogue should always be able to use their reaction for Uncanny Dodge unless they're physically restrained from doing so. Not to allow it would be a bigger balance issue. A rogue in single combat against a fighter is still in a sub-optimal position, as you've pointed out, as the fighter still has 2 attacks to the rogue's 1, and the rogue can only Uncanny Dodge once per turn. The fighter in that scenario is just unlucky that their second attack missed, but you could also make the case that they're lucky that on the turn he only landed one attack, it was at least a crit, so the rogue still took more damage than they would have from a regular hit, UD or no. It's still a net positive for the fighter.
As a DM I use NPCs built as PCs for villains all the time. It functions the same as PvP for comparison sake though.
I thought of a few things that could come up in combat where rules start to overlap and be messy, so where would you put rulings for these things. Feel free to vote and comment or just one or the other or just read to see what people say. The polls are as follows
Mobile "Don't Provoke oppurtunity attack if you attack a creature with melee" vs. Sentinel "Creature provoke oppurtunity attack, even if they Disengage, when they leave your reach."
Level 5 Champion Fighter Hits with a Critical on the first attack and misses on the second vs. Level 5 Thief Rogue uses uncanny dodge to take away half the damage.
Shield spell react automatically to enemy attacks you taking away reaction vs. Shield spell reacting only when you would have taken damage (can still use reactions if they miss).
Level 1 Variant Human Fighter with Magic Initiate Feat vs. Level 1 Variant Human Wizard with Weapon Master Feat.
Mobile "wins". Sentinel prevents the "Disengage" action from doing anything, but mobile is a different thing, Sentinel has no interaction with the mobile feat.
I don't even get the question you have champion fighter and rogue. The champion fighter gets a crit, so they roll double dice to figure out damage. The rogue can use their reaction uncanny dodge to take half of that damage instead of the full amount, if they can see the attacker. How does the second attack factor into it at all? The rogue has to decide whether to use their reaction for uncanny dodge after the first attack, before they know whether the second one hit or not. I don't even understand what you're asking about.
Shield specifies that you can cast it when you are hit by an attack or Magic Missile. It can only be used if you've been hit by the attack or targeted by the MM. If you haven't been hit (such as because the enemy did not roll well enough to hit your normal AC) then you can't cast Shield.
What are you even asking about between fighter with magic initiate and wizard with weapon master? Which one we "like" more?
To the last poll: a fighter with Magic Initiate is still a fighter, but he can pick up several helpful utility cantrips and long lasting spells like Goodberry that improve his overall performance.
A wizard who can use a sword is still a wizard who has no business in melee. He still only gets one attack that won't hit due to his poor physical ability scores. The notable exception are Bladesingers. But they are a) not Variant Humans and b) get weapon proficiency at level 2 anyways, in addition to their elven weapon proficiency.
So for the fighter Magic Initiate is a valuable feat that improves his performance, while for the wizard Weapon Master is pretty much useless.
In regards to the point claiming that Uncanny Dodge working on a crit would make crits useless against rogues, it makes me wonder how much PvP combat you're running? Dnd is not a very robust PvP game, so judging player abilities' effectiveness against each other is sort of a moot point. Most of these abilities were designed to work in tandem with other characters, not against them. Without the ability to Uncanny Dodge, rogues (with their limited defensive capabilities and small hit dice) would not be able to stay in fights alongside their more heavily armored party members, and with an entire class unable to compete with others, the game becomes imbalanced to the point where, why even bother picking rogue?
So yes, a rogue should always be able to use their reaction for Uncanny Dodge unless they're physically restrained from doing so. Not to allow it would be a bigger balance issue. A rogue in single combat against a fighter is still in a sub-optimal position, as you've pointed out, as the fighter still has 2 attacks to the rogue's 1, and the rogue can only Uncanny Dodge once per turn. The fighter in that scenario is just unlucky that their second attack missed, but you could also make the case that they're lucky that on the turn he only landed one attack, it was at least a crit, so the rogue still took more damage than they would have from a regular hit, UD or no. It's still a net positive for the fighter.
So on the second one with the rogue and fighter, maybe to avoid PvP we just say the monster attacks twice and crits on one of them. Since the monster is attacking the rogue twice and the rogue only has one reaction they have to decide whether or not to UD the first attack before knowing the result of the second? Sure other monsters could also attack the rogue when he uses his reaction but in regards to the same action taken by 1 monster since all those attacks are part of the one action the rogue has decide whether the first one is halfed before knowing the second? What if first attack dealt 3 damage and the other dealt 10? The rogue would take 11 damage in the end not knowing the second would be greater instead of 8?
The original question just had to do with whether UD halves a critical hit or not, but this provides a new question. Would a rogue react to half one attack that hits after the attack, or would they react to half only one of the attacks that hits after the action?
Yes, of course. Because the monster can make different decisions based on whether the first attack kills the rogue or not (and whether the rogue used UD on the first attack or not.) Turns happen sequentially, monsters get to do things in order just like players do.
The rogue doesn't even know whether there will *be* a second attack - it might depend on how much damage he takes!
Yep. The sequence is:
1) The rogue is attacked and gets dealt some damage.
2) The rogue gets to choose whether to use their reaction to react to that damage in some way, if they can - UD for example.
3) Then the monster gets to *decide* what to do next. It might be attacking the rogue again, it might be attacking someone else, it might be doing something totally different. The first attack might have changed the situation.
Yes, UD halves a critical hit.
Uncanny dodge says " when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you." So it applies to one attack, and happens as soon as an attacker you can see hits you with the attack, before finding out what else the attacker is going to do on their turn.
"Then the monster gets to *decide* what to do next. It might be attacking the rogue again, it might be attacking someone else, it might be doing something totally different. The first attack might have changed the situation."
So a creature with multiattack can use the same action to hit a different creature? This isn't assuming the creature hit by the initial attack dies; I mean yeah that would make sense to have the second attack carry on to another creature within 5 foot range if melee, 10 foot if reach, and so on, but supposing the creature didn't die to the first attack which is still part of the monsters action could they decide mid-action to switch over to another creatures. Could players do this as well where the fighter at level 5 uses multiattack and action surge in combat to attempt a hit against 4 different creatures and not two attacks against one as the first action and then two attacks on a second as part of their second action, being able to change targets because it's a new action and not a new attack?
If attacks are separated from actions and not part of an action then we say that the monster can decide whether to try and hit the rogue again after the rogue UD their first attack, but if the attacks cannot be separated from the action, like how a character can't use an action to drink half a potion to restore just half the effects and give it to their buddy to drink the other half and take half the effect, then the attack action likewise couldn't be halfed. Even in the case of downed enemy there is the idea of negative hit points where the first hit brings them down to zero or even passed zero and the second hit goes further into the negative, possibly perma-death that creature.
In the end UD still only halves one attack that hits, if the creature has action surge or a bonus action or even just other creatures in the round come to hit the rogue, they can't react any more. Now as the DM you would know which of your monsters will attack the rogue, what you don't know is if they will hit or not, no one can know that until the roll is made. Knowing a second monster will move in to attack the rogue where the first monster is but the rogue doesn't know this they just deal with the first monster who after being hit sneak attack is going to use its action to attack the rogue taking the multiattack action to deal 2 hits. Both times hits, and while the rogue doesn't know you have plans to send the second monster to attack as well only gets access to the information the dice gave to them, whether the hit succeeded or not. The rogue can't react before knowing the roll because it's only when a result is shared that they know if they got hit or not and only by being hit can they react. So rather than having the rogue say they want to uncanny dodge that action but wait to see which attacks will hit and which won't (or deciding if there is one of the two or three or however many multiattacks that action takes that they decide to half just one of them) it's only upon being first hit that they either use UD or they give it up.
See that's kind of the idea of this poll was to look at peoples thoughts and to break things down further because different people have different interpretations as how they see the rules whether it's through their experiences playing the game or running it or reading sources or wherever it might be. Some of the information isn't ironclad so it's important to half discussions outside of a game so that they don't get bogged down by all this jargon. In the end with rule 0 the DM interprets the rules however they want so it doesn't really matter what anyone on here says, but different perspectives can be shared and since no one is the DM in here no one is the arbiter of how things works....
Though I guess I would concede an argument if a tweet from Jeremy Crawford is shared to at least say "Here is what the devs say, this was there intention" which is fair, it says if you want to play the game your own way go ahead but if you want to play RAW and RAI then this is how it's done.
None of these situations are at all ambiguous in the rules.
1) Mobile > Sentinel
Mobile: "When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not."
Sentinel: "Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach."
Sentinel only mentions Disengage, not any other effects while mobile very explicitly states you don't provoke op attacks.
2) Thief can choose to use Uncanny Dodge on any hit. They do NOT know whether the next hit would be a crit or not. Each hit of an Attack action or Multiattack has to be resolved separately in order. The rules explicitly allow players with extra attack to move between attacks of the attack action and as a DM I would apply this to creatures with multiattack as well. Uncanny Dodge works as well on crits as anything else.
3) Shield spell explicitly states that it can be used when HIT. I'm not sure what you are even asking with this question unless it is about some sort of homebrew.
4) This question is also unclear as to what you are looking for ... MI fighter makes much more sense than a WM wizard if you are considering melee. Fighter has better armor, hit points, shield, second wind ... if melee is the goal the fighter is the best choice. However, you don't specify level or anything else. If you want a wizard with a bit of melee capability then a Bladesinger makes far more sense than taking the WM feat.
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Basically, none of the situations you have described are ambiguous or difficult to figure out what the rules say. There are far more contentious options out there :)
For example: What is total cover? Does a Wall of Force provide total cover preventing the casting of spells that only require you to see your target? Does a thin fragile window pane provide total cover? Does the thin window pane prevent the casting of spells or the firing of a crossbow bolt through the glass? This is an issue where you will get very divisive answers :)
Yes... that's how attacks work. In no stage of combat do you have to declare a single target for your turn.
Each attack a creature has is essentially approached as a unique instance. A fighter cornered by two gnolls might choose to try and focus one of them down with both attacks, or might choose to attack each one once. That's always they're choice and nothing ever prevents them from doing that.
Now, let's go back to your rogue. A monster with, say 3 attacks, can attack the rogue with all three (regardless of which ones hit), and let's say the first one is a crit that gets Uncanny Dodged-- the monster *can* choose to continue with more attacks on the rogue, it *can* choose to spread the rest of its attacks onto other party members, it *can* choose to give up attacking altogether and whine about getting UD'd. An attacker makes this choice *every time* they attack, and is more influenced by how smart or strategic the DM wants this monster to behave.
Maybe the rogue UD's a crit on the first attack from a Smart Monster who knows that they can't do that again, and decides to press their advantage by hitting them another two times.
Maybe the rogue UD's a Stupid Monster who thinks "aaagh! Puny creature hard to hit! I kill sumfing else!" and tries to hit something else with their subsequent attacks. Both are valid responses that are entirely up to how the DM is running the monster, and how difficult the DM wants the encounter to be.
All that, however, is irrelevant to your original question. There is zero ambiguity in the answer to your original question: a rogue can always spend their reaction to UD, even against crits, regardless of whatever else happens that turn. Wether they decide to wait and see if they're about to get hit harder (or at all) before using their reaction, or save it to use on a bigger attacker that turn, is entirely up to player discretion.
Of those options, the rules are completely clear on the first three (sentinel only negates disengage, uncanny dodge has no restriction on crits, the trigger for shield is being hit) and the fourth is a judgment call between two bad options (a fighter with magic initiate should take abilities that don't depend on their spellcasting stat, a wizard is not going to touch weapon attacks after level 4).
So I did some further research and people are right about changing targets in multiattack and rogues not deciding which hit to UD.
People can continue commenting on here, I am pretty much going to abandon this post but don't feel to remove it (not even sure I can).
Another person on here had better examples of ambiguous rulings, or could be about something else, not all sure.
Anyway feel free to rate this any number of jars of Newman's Own marinara sauce out of 5.
My answer to the original question was the rogue could half the damage regardless of whether it was a crit or not. My answer to this new question would be that the rogue would have to choose to uncanny dodge the first attack before seeing the results of the second. I've yet to play at a table with a rogue high enough to have uncanny dodge, so I'm not sure how necessary using it on a 3 damage would be, but I would assume that it would only stress me if the attack would knock me unconcious, particularly since I'd only be reducing the damage by one. I'd have to know what my health looked like and more of the situation to know whether I'd try to half the 10. Considering that 2 hits for 13 damage is probably not a good sign against a rogue, I'd probably opt to half the 10.
As a DM I use NPCs built as PCs for villains all the time. It functions the same as PvP for comparison sake though.
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