The cage created by forcecage is invisible, which adds the complication that disintegrate can only be used against it by casters that can overcome the invisibility. Aside from that, does this also mean that the cage's location is unknown to creatures that can't overcome the invisibility? Could the situation arise where the enemy casts forcecage and the players don't know where the cage is, and then the wizard casts fireball but it hits the forcecage in front of him and it blows up in his face? (Attempting to target a point of origin on the other side of total cover has it appear on the near side instead).
According to Jeremy Crawford, Disintegrate makes a visibility exception for invisible barriers like Forcecage, so the caster would need to know where the cage was, but wouldn't need to actually see it. You may have used Detect Magic, seen someone else walk into it, or perhaps you are personally touching the cage. [/shrug]
Whether or not a wizard can cast Fireball and miss.... that's up to the DM. Generally speaking, you can't target something with total cover. There is no justification for this, you just can't, even if the cover is a thin pane of glass. Your DM may handwave these interactions at the table.
The cage created by forcecage is invisible, which adds the complication that disintegrate can only be used against it by casters that can overcome the invisibility.
There really is no justification in the RAW for being able to target a Forcecage with the Disintegrate spell. They clearly meant it to work but failed miserably when writing the spell descriptions. The sentence that that JC says create an exception doesn't actually say it does so, all it does is list the things that can be targeted. In that list is "a creation of magical force" but it also has "a creature" and "an object" but no mention of either of them being treated differently from the others and no mention about not needing to be seen which is required by the sentence before. So if an invisible Forcecage can be targeted then the same should hold for an invisible creature or object but then the part where it says "a target that you can see" becomes completely irrelevant and that's not how spell descriptions are meant to work. I might also note that the tweet that @Memnosyne linked to haven't made it into the sage advice compendium so its RAI footing is somewhat shaky.
Note that I'm not saying that it would be wrong for a DM to allow it (it clearly seems meant to work), I'm just saying beware of the wording of the spell and check with your DM beforehand.
Aside from that, does this also mean that the cage's location is unknown to creatures that can't overcome the invisibility?
As long as you don't witness something interacting with the cage then you likely should not know its location. Of course if you have Truesight or some similar sense then you would also see it.
Could the situation arise where the enemy casts forcecage and the players don't know where the cage is, and then the wizard casts fireball but it hits the forcecage in front of him and it blows up in his face? (Attempting to target a point of origin on the other side of total cover has it appear on the near side instead).
@Memnosyne is correct in that you can't target something/someone that have total cover from you. But then again the rules do also say that if you try to target a spot you can't see and there is something blocking that point (i.e it has total cover) then the spell still goes off but goes off at the spot of the blockage so some DM adjudication is clearly needed.
If you can't see invisibile then you don't know where the cage is. If you didn't identify the spell the opposing mage cast then you might not even know there is a force cage present.
My reading is that the rules are clear - when you cast fireball at your foes you will get a horrible surprise when it goes off a few feet in front of you.
The rules are clear on disintegtate as well. If you can't see the wall (which applies to forcecage as well as wall of force and similar) then you can't disintegrate it. In my games I houserule a special case for using disintegrate on those spells, but you have to be explicitly targeting the wall and you have to, of course, know the existance and location of the wall.
@Memnosyne is correct in that you can't target something/someone that have total cover from you. But then again the rules do also say that if you try to target a spot you can't see and there is something blocking that point (i.e it has total cover) then the spell still goes off but goes off at the spot of the blockage so some DM adjudication is clearly needed.
Just to cite the rule you are mentioning :)
"A Clear Path to the Target
To target something, you must have a clear path to it, so it can’t be behind total cover.
If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction."
The rule above makes it clear that you CAN target a point that you can't see that has total cover (can't see and path is obstructed) with an area of effect spell, when this happens the point of origin comes into being on the near side of that obstruction. So, in the case of a fireball, you might be able to cast it at a point you can't see but if there is a wall of force in between, the origin of the spell comes into effect on the near side of that obstruction. It seems to be a case of specific beating general and in the case described above, the wizard could end up hitting themselves with a fireball when it detonates unexpectedly against the wall of force (assuming that the caster could not see the point they were targeting).
So, a wizard can't map the locations of walls in a room or the location of a wall of force by saying "I try to cast fireball at this point", DM says "not a valid target", Wizard says "ok ... must be a wall in the way" etc.
P.S. It is a completely separate and contentious discussion as to whether a Wall of Force actually provides total cover or not ... this is also up to DM adjudication.
According to Jeremy Crawford, Disintegrate makes a visibility exception for invisible barriers like Forcecage, so the caster would need to know where the cage was, but wouldn't need to actually see it.
I don't know how Jeremy Crawford concluded that, but it's completely wrong. No such exclusion of magical force walls exists.
"A thin green ray springs from your pointing finger to a target that you can see within range. The target can be a creature, an object, or a creation of magical force, such as the wall created by wall of force."
According to Jeremy Crawford, Disintegrate makes a visibility exception for invisible barriers like Forcecage, so the caster would need to know where the cage was, but wouldn't need to actually see it.
I don't know how Jeremy Crawford concluded that, but it's completely wrong. No such exclusion of magical force walls exists.
"A thin green ray springs from your pointing finger to a target that you can see within range. The target can be a creature, an object, or a creation of magical force, such as the wall created by wall of force."
He concluded that by being the lead rules designer for WoTC. He reads it with the context of what what was meant at the time it was written.
Keep in mind that if you want to be a rules literalist, then spells like See Invisibility and True Seeing won't let you see Wall of Force because it's a "creation of magical force" and not an object. Ergo, Disintegrate can never destroy Wall of Force, even though it is specifically listed in both spells as being able to do so.
Honestly whether you interpret the targeting requirements described for Disintegrate spell as allowing a Forcecage or not seems to me to be a moot question.
Even if a Forcecage or Wall of Force cannot be directly targeted, you can target an object or creature behind it. If something cannot be seen then it also cannot conceal something behind it, which is the RAW requirement for an obstacle to provide Total Cover.
Honestly the targeting and cover rules in 5e are a mess. They seem to go out of their way to avoid using terms like Line of Effect and Line of Sight, I suppose in an effort to be more intuitive, but they are a mess when put to close scrutiny.
Honestly whether you interpret the targeting requirements described for Disintegrate spell as allowing a Forcecage or not seems to me to be a moot question.
Even if a Forcecage or Wall of Force cannot be directly targeted, you can target an object or creature behind it. If something cannot be seen then it also cannot conceal something behind it, which is the RAW requirement for an obstacle to provide Total Cover.
Honestly the targeting and cover rules in 5e are a mess. They seem to go out of their way to avoid using terms like Line of Effect and Line of Sight, I suppose in an effort to be more intuitive, but they are a mess when put to close scrutiny.
Such is the cost of a "Plain English" ruleset. The rules were never meant to be scrutinized with a magnifying lens
The RAW for total cover is obviously intended to be consistent with the rest of the section, so the use of "concealed" shouldn't be interpreted strictly. People get so hung up on things that genuinely don't matter, within rules that were never more than guidelines.
According to Jeremy Crawford, Disintegrate makes a visibility exception for invisible barriers like Forcecage, so the caster would need to know where the cage was, but wouldn't need to actually see it.
I don't know how Jeremy Crawford concluded that, but it's completely wrong. No such exclusion of magical force walls exists.
"A thin green ray springs from your pointing finger to a target that you can see within range. The target can be a creature, an object, or a creation of magical force, such as the wall created by wall of force."
He concluded that by being the lead rules designer for WoTC. He reads it with the context of what what was meant at the time it was written.
Keep in mind that if you want to be a rules literalist, then spells like See Invisibility and True Seeing won't let you see Wall of Force because it's a "creation of magical force" and not an object. Ergo, Disintegrate can never destroy Wall of Force, even though it is specifically listed in both spells as being able to do so.
Is it really not an object? I know "creation of magical force" is listed in the disintegrate spell separately from "object", but is there an actual rule that cements that creations of magical force don't count as objects?
Knowing a target's location doesn't make it valid target you can see. It goes for invisible creature that aren't hidden as much as invisible creation of magical force. Disintegrate can't target anything it cannot see by RAW, including a Forcecage. While the spell clarify what the target can be in terms of creation of magical force, the first requirement is that it must be a target that you can see within range.
If the Dev intended it to work, RAI doesn't align with RAW.
"A thin green ray springs from your pointing finger to a creature, an object, or a creation of magical force that you can see within range."
A thin green ray springs from your pointing finger to a target that you can see within range. The target can be a creature, an object, or a creation of magical force, such as the wall created by wall of force.
Time to complicate this discussion further. Forcecage doesn't actually list disintegrate as as an exemption to its indestructibility. It also doesn't say it's indestructible, so a caster that could somehow see the cage could still disintegrate it (according to the disintegrate spell itself). But other damage sources? A fighter going bonk with a weapon? All we really know is that disintegrate works but dispel magic doesn't.
Many would claim that the spell doesn't give the cage hit points, so it must be indestructible. But the game elects not to provide hit points for lots of things (wells, daggers, etc.). These things do, of course, have hit points; the GM needs to come up with them on the spot, likely using the guidelines in DMG chapter 8.
Is it really not an object? I know "creation of magical force" is listed in the disintegrate spell separately from "object", but is there an actual rule that cements that creations of magical force don't count as objects?
There are a lot of things that simply aren't spelled out. If we want to follow a strict reading, this list separating force constructs from objects is the only precedent we have. Most things that are invisible have a "true form" that can be discerned, but force constructs are fundamentally immaterial, so it could be that their "true form" is nonexistent, so there is simply nothing to see (Though Blindsight might circumvent this by interacting with it's physical effects, rather than its substance).
What we know is Disintegrate is intended to work against Wall of Force, so there must be a path to do so. We can either handwave the classification of Wall of Force to count as an object, allowing it to be perceived directly through magic, or we can say that Disintegrate is an anathema for Wall of Force and accept an operational exception (Like Light and Darkness spells cancelling each other out). Wall of Force is a 5th level spell, Disintegrate is a 6th level spell, and Forcecage is a 7th level spell, so I'd be perfectly happy houseruling that Disintegrate works outright against Wall of Force, but has no affect on Forcecage unless it is cast at 8th level. (Or treating it like Counterspell with a spell check to overcome higher level force constructs.)
Edit: Mike Mearls has said Wall of Force would appear transparent via See Invisibility or True Sight, which seems to confirm that it would still be invisible. (Transparent versus translucent). However, transparency has some wiggle room as a mismatch of refractive indices could allow it to be perceived through distortions. The game doesn't get into these kinds of details though.
The cage created by forcecage is invisible, which adds the complication that disintegrate can only be used against it by casters that can overcome the invisibility. Aside from that, does this also mean that the cage's location is unknown to creatures that can't overcome the invisibility? Could the situation arise where the enemy casts forcecage and the players don't know where the cage is, and then the wizard casts fireball but it hits the forcecage in front of him and it blows up in his face? (Attempting to target a point of origin on the other side of total cover has it appear on the near side instead).
[SageAdvice]
According to Jeremy Crawford, Disintegrate makes a visibility exception for invisible barriers like Forcecage, so the caster would need to know where the cage was, but wouldn't need to actually see it. You may have used Detect Magic, seen someone else walk into it, or perhaps you are personally touching the cage. [/shrug]
Whether or not a wizard can cast Fireball and miss.... that's up to the DM. Generally speaking, you can't target something with total cover. There is no justification for this, you just can't, even if the cover is a thin pane of glass. Your DM may handwave these interactions at the table.
There really is no justification in the RAW for being able to target a Forcecage with the Disintegrate spell. They clearly meant it to work but failed miserably when writing the spell descriptions. The sentence that that JC says create an exception doesn't actually say it does so, all it does is list the things that can be targeted. In that list is "a creation of magical force" but it also has "a creature" and "an object" but no mention of either of them being treated differently from the others and no mention about not needing to be seen which is required by the sentence before. So if an invisible Forcecage can be targeted then the same should hold for an invisible creature or object but then the part where it says "a target that you can see" becomes completely irrelevant and that's not how spell descriptions are meant to work. I might also note that the tweet that @Memnosyne linked to haven't made it into the sage advice compendium so its RAI footing is somewhat shaky.
Note that I'm not saying that it would be wrong for a DM to allow it (it clearly seems meant to work), I'm just saying beware of the wording of the spell and check with your DM beforehand.
As long as you don't witness something interacting with the cage then you likely should not know its location. Of course if you have Truesight or some similar sense then you would also see it.
@Memnosyne is correct in that you can't target something/someone that have total cover from you. But then again the rules do also say that if you try to target a spot you can't see and there is something blocking that point (i.e it has total cover) then the spell still goes off but goes off at the spot of the blockage so some DM adjudication is clearly needed.
If you can't see invisibile then you don't know where the cage is. If you didn't identify the spell the opposing mage cast then you might not even know there is a force cage present.
My reading is that the rules are clear - when you cast fireball at your foes you will get a horrible surprise when it goes off a few feet in front of you.
The rules are clear on disintegtate as well. If you can't see the wall (which applies to forcecage as well as wall of force and similar) then you can't disintegrate it. In my games I houserule a special case for using disintegrate on those spells, but you have to be explicitly targeting the wall and you have to, of course, know the existance and location of the wall.
Just to cite the rule you are mentioning :)
"A Clear Path to the Target
To target something, you must have a clear path to it, so it can’t be behind total cover.
If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction."
The rule above makes it clear that you CAN target a point that you can't see that has total cover (can't see and path is obstructed) with an area of effect spell, when this happens the point of origin comes into being on the near side of that obstruction. So, in the case of a fireball, you might be able to cast it at a point you can't see but if there is a wall of force in between, the origin of the spell comes into effect on the near side of that obstruction. It seems to be a case of specific beating general and in the case described above, the wizard could end up hitting themselves with a fireball when it detonates unexpectedly against the wall of force (assuming that the caster could not see the point they were targeting).
So, a wizard can't map the locations of walls in a room or the location of a wall of force by saying "I try to cast fireball at this point", DM says "not a valid target", Wizard says "ok ... must be a wall in the way" etc.
P.S. It is a completely separate and contentious discussion as to whether a Wall of Force actually provides total cover or not ... this is also up to DM adjudication.
I don't know how Jeremy Crawford concluded that, but it's completely wrong. No such exclusion of magical force walls exists.
"A thin green ray springs from your pointing finger to a target that you can see within range. The target can be a creature, an object, or a creation of magical force, such as the wall created by wall of force."
He concluded that by being the lead rules designer for WoTC. He reads it with the context of what what was meant at the time it was written.
Keep in mind that if you want to be a rules literalist, then spells like See Invisibility and True Seeing won't let you see Wall of Force because it's a "creation of magical force" and not an object. Ergo, Disintegrate can never destroy Wall of Force, even though it is specifically listed in both spells as being able to do so.
Honestly whether you interpret the targeting requirements described for Disintegrate spell as allowing a Forcecage or not seems to me to be a moot question.
Even if a Forcecage or Wall of Force cannot be directly targeted, you can target an object or creature behind it. If something cannot be seen then it also cannot conceal something behind it, which is the RAW requirement for an obstacle to provide Total Cover.
Honestly the targeting and cover rules in 5e are a mess. They seem to go out of their way to avoid using terms like Line of Effect and Line of Sight, I suppose in an effort to be more intuitive, but they are a mess when put to close scrutiny.
Such is the cost of a "Plain English" ruleset. The rules were never meant to be scrutinized with a magnifying lens
The RAW for total cover is obviously intended to be consistent with the rest of the section, so the use of "concealed" shouldn't be interpreted strictly. People get so hung up on things that genuinely don't matter, within rules that were never more than guidelines.
Is it really not an object? I know "creation of magical force" is listed in the disintegrate spell separately from "object", but is there an actual rule that cements that creations of magical force don't count as objects?
Knowing a target's location doesn't make it valid target you can see. It goes for invisible creature that aren't hidden as much as invisible creation of magical force. Disintegrate can't target anything it cannot see by RAW, including a Forcecage. While the spell clarify what the target can be in terms of creation of magical force, the first requirement is that it must be a target that you can see within range.
If the Dev intended it to work, RAI doesn't align with RAW.
As written Disintegrate effectively translate as:
"A thin green ray springs from your pointing finger to a creature, an object, or a creation of magical force that you can see within range."
A thin green ray springs from your pointing finger to a target that you can see within range. The target can be a creature, an object, or a creation of magical force, such as the wall created by wall of force.
Time to complicate this discussion further. Forcecage doesn't actually list disintegrate as as an exemption to its indestructibility. It also doesn't say it's indestructible, so a caster that could somehow see the cage could still disintegrate it (according to the disintegrate spell itself). But other damage sources? A fighter going bonk with a weapon? All we really know is that disintegrate works but dispel magic doesn't.
Many would claim that the spell doesn't give the cage hit points, so it must be indestructible. But the game elects not to provide hit points for lots of things (wells, daggers, etc.). These things do, of course, have hit points; the GM needs to come up with them on the spot, likely using the guidelines in DMG chapter 8.
There are a lot of things that simply aren't spelled out. If we want to follow a strict reading, this list separating force constructs from objects is the only precedent we have. Most things that are invisible have a "true form" that can be discerned, but force constructs are fundamentally immaterial, so it could be that their "true form" is nonexistent, so there is simply nothing to see (Though Blindsight might circumvent this by interacting with it's physical effects, rather than its substance).
What we know is Disintegrate is intended to work against Wall of Force, so there must be a path to do so. We can either handwave the classification of Wall of Force to count as an object, allowing it to be perceived directly through magic, or we can say that Disintegrate is an anathema for Wall of Force and accept an operational exception (Like Light and Darkness spells cancelling each other out). Wall of Force is a 5th level spell, Disintegrate is a 6th level spell, and Forcecage is a 7th level spell, so I'd be perfectly happy houseruling that Disintegrate works outright against Wall of Force, but has no affect on Forcecage unless it is cast at 8th level. (Or treating it like Counterspell with a spell check to overcome higher level force constructs.)
Edit: Mike Mearls has said Wall of Force would appear transparent via See Invisibility or True Sight, which seems to confirm that it would still be invisible. (Transparent versus translucent). However, transparency has some wiggle room as a mismatch of refractive indices could allow it to be perceived through distortions. The game doesn't get into these kinds of details though.