Disintegrate and Polymorph both have effects that happen when the subject is reduced to 0 hit points.
Disintegrate says:
If this damage reduces the target to 0 hit points, it is disintegrated.
Polymorph says:
The transformation lasts for the duration, or until the target drops to 0 hit points or dies.
Which takes precedence? Does Disintegrate reduce the a Polymorphed target to dust first or does the creature revert to its regular form first?
Apparently, RAI is in opposition to my other answer per a tweet from Mr Crawford (Credit to @Airatome for this tweet):
Jeremy Crawford
@JeremyECrawford
The intent is that a druid using Wild Shape is disintegrated if the druid, not the beast form, drops to 0 hp. #DnD
3:10 PM - 17 Sep 2015Joshua Maxman
@Orethalion
@JeremyECrawford What happens if a wild shaped druid is reduced to 0 by disintegrate? Does he revert to normal physical form?
6:20 PM - 16 Sep 2015
This means that if you are under Polymorph, Wildshape, Shapechange, or True Polymorph, you'd trigger the "revert to your original form" condition once Disintegrate reduced you to 0 HP, and take any additional damage to your original form in excess of what it took to reduce you to zero from Disintegrate.
True Polymorph and Polymorph both say ...
... target assumes the hit points of its new form, and when it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn’t reduce the creature’s normal form to 0 hit points, it isn’t knocked unconscious.
Wild Shape
When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. For example, if you take 10 damage in animal form and have only 1 hit point left, you revert and take 9 damage. As long as the excess damage doesn’t reduce your normal form to 0 hit points, you aren’t knocked unconscious.
Shapechange
When you revert to your normal form, you return to the number of hit points you had before you transformed. If you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. As long as the excess damage doesn’t reduce your normal form to 0 hit points, you aren’t knocked unconscious.
Per @Airatome's comment on Mr Crawford's tweet:
I can't find a way to bring up the ENTIRE conversation, but it was noted this applies to Polymorph target's as well.
Short Answer
The target is Disintegrated/Dust, because this specific case of "reducing to 0 hit points" due to this spell is unique to this spell effect, versus the general case of what happens when a creature is reduced to 0 hit points.
If this damage reduces the target to 0 hit points, it is disintegrated. A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a true resurrection or a wish spell.
The Sage Advice Compendium supports this answer from a RAW perspective:
What happens if a druid using Wild Shape is reduced to 0 hit points by disintegrate? Does the druid simply leave beast form?
The druid turns to dust, since the spell disintegrates you the instant you drop to 0 hit points. That’s the literal interpretation of the rules (RAW).In contrast, the intent (RAI) is that a druid isn’t considered to be at 0 hit points for the purposes of an effect like disintegrate until the druid’s normal form is reduced to 0 hit points.
There is some tension between this spell's result at 0 HP and the other shape changing methods.
Polymorph (both regular and true), Shapechange, and Wildshape all have a consistent rule of reverting to original form when 0 HP are reached, and can be summarized as follows:
If you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. As long as the excess damage doesn’t reduce your normal form to 0 hit points, you aren’t knocked unconscious.
This answer favors Disintegrate because it bypasses, or renders null and void, considerations of the "unconscious" state at 0 HP. This links in to the "what happens when you are reduced to 0 HP" rule. You do not get the chance at a death saving throw, nor to stabilize in the disintegrate case.
When you drop to 0 hit points, you either die outright or fall unconscious, as explained in the following sections.
The Disintegrate result overwrites the "fall unconscious" effect for the targeted creature. The tension between the specific rules, over the general 0 HP rule, exists because Polymorph spells also overwrite unconscious because you don't fall unconscious when you revert to old form if the damage taken doesn't also exceed your normal HP.
Why favor Disintegrate? The transition phase still needs a creature to make that happen. With Disintegrate's spell effect, there isn't a creature there any longer to reform into an original form. It seems valid to rule that disintegrate overrides the transitional form because it also destroys all non-magical equipment. All of that is part of the pile of dust. A pile of dust is not a creature. Another nod to Disintegrate's power, and thus why it should take precedence, is that it takes a Wish or True Resurrection to come back from that. (That's powerful magic).
Apparently, RAI is in opposition to my argument per a tweet from Mr Crawford (Credit to @Airatome for this tweet):
Jeremy Crawford @JeremyECrawford Jeremy Crawford Retweeted Joshua Maxman
The intent is that a druid using Wild Shape is disintegrated if the druid, not the beast form, drops to 0 hp. #DnD Jeremy Crawford added, Joshua Maxman @Orethalion
@JeremyECrawford What happens if a wild shaped druid is reduced to 0 by disintegrate? Does he revert to normal physical form? Retweet 3:10 PM - 17 Sep 2015
Extended Answer
Two cases:
Player Characters
See above
All other Creatures
Since all other creatures die at 0 HP (unless the DM makes a special case to have them follow PC rules) that shouldn't influence the above.
Monsters and Death
Most GMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws. Mighty villains and special nonplayer characters are common exceptions; the GM might have them fall unconscious and follow the same rules as player characters.
Other considerations
The spell effect changes a creature into a non-creature substance: dust. It would be consistent to rule that no matter what shape change or polymorph power was being used, Disintegrate is a special case and the Specific beats General argument should prevail for it.
Why not have it work the other way?
Per the tweet linked, it appears that both Polymorphed cretaures and Wildshaped Druids would revert, rather than turn to dust, as RAI.
If one rules that way, True Polymorph, Polymorph, Wildshape, and Shapechange become special defenses against the Disintegrate spell, even though these spells already provide the added benefit of an extra HP pool for the creature in that shape. Some will argue that this would move the "balance" in the direction of "unbalanced" or "too powerful." Others won't.
The tweet favors the "specific" of polymorph/Wild Shape/Shapechange/True Polymorph, rather than the specific of disintegrate, requiring the exhaustion of both HP pools before being turned to dust by that spell.
I can't find a way to bring up the ENTIRE conversation, but it was noted this applies to Polymorph target's as well.
– Airatome Jan 13 '16 at 17:54You could see it as one of two things, though they both have very similar end results:
a) The body simply disintegrates, and stays that way. The Polymorph would not be able to reverse the disintegration spell since it isn't a True Resurrection Spell, or a Wish Spell.
(Taken from the Disintegrate spell)
A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a true resurrection or a wish spell.
or b) The body polymorphs back into its original form, then it disintegrates. The entire basis of the Disintegrate spell is to turn something to ash, and there doesn't seem to be anything in the polymorph spell that would counter that or stop the disintegration.
The differences are entirely for the roleplay description of the monster as it disintegrates, and the size of the ash-pile.
The answer is in the Polymorph spell's description. "The transformation lasts for the duration, or until the target drops to 0 hit points or dies."
Thus reaching 0 HP will cause you to revert as will death. Yes, Disintegrate turns you to dust. It kills you. According to the Polymorph spell description, death will revert you to your original form with your original HP (with extra damage carried over).
There is nothing in the rules that says you cannot revert to your original form if you are no longer a "creature." The other answers seem to assume that as a rule and are forgetting that death in animal form is stated to also revert you, not just dropping to 0 HP.
