There are a large number of questions on this site and online regarding the interaction between effects that cause death at 0 hit points and features that prevent death at 0 hit points.
Effects that cause death if they reduce a character to 0 hit points include:
- Disintegrate spell (original): "If this damage reduces the target to 0 hit points, it is disintegrated."
- Disintegrate spell (PH Errata 2.0): "The target is disintegrated if this damage leaves it with 0 hit points."
- Beholder's Disintegration Ray: "If this damage reduces the target to 0 hit points, its body becomes a pile of fine gray dust."
- Beholder's Death Ray: "The target dies if the ray reduces it to 0 hit points."
- A published adventure* contains a large number of effects with language like "If this damage reduces the creature's hit points to 0, it is crushed to a pulp."
- Instant Death: "When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your high point maximum."
Effects that can prevent character death when a character is reduced to 0 hit points include (HT to this question for some of these):
- Half-orc's Relentless Endurance: "When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead."
- Barbarian's Relentless Rage: "If you drop to 0 hit points while you're raging and don’t die outright, you can make a DC 10 Constitution saving throw. If you succeed, you drop to 1 hit point instead."
- Druid's Wild Shape: "You automatically revert if you fall unconscious, drop to 0 hit points, or die. […] When you revert to your normal form, you return to the number of hit points you had before you transformed."
- Death Ward spell: "The first time the target would drop to 0 hit points as a result of taking damage, the target instead drops to 1 hit point, and the spell ends."
- Polymorph spell: "The transformation lasts for the duration, or until the target drops to 0 hit points or dies. […] When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed."
The interaction between these effects has been a persistent question. Sage Advice used to have the following ruling:
If the damage from disintegrate reduces a half-orc to 0 hit points, can Relentless Endurance prevent the orc from turning to ash? If disintegrate reduces you to 0 hit points, you’re killed outright, as you turn to dust. If you’re a half-orc, Relentless Endurance can’t save you.
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.
But then the wording of the disintegrate spell changed, and in Sage Advice 2.0, the following ruling appeared:
[NEW] If the damage from disintegrate reduces a half- orc to 0 hit points, can Relentless Endurance prevent the orc from turning to ash? Yes. The disintegrate spell turns you into dust only if the spell's damage leaves you with 0 hit points. If you're a half-orc, Relentless Endurance can turn the 0 into a 1 before the spell can disintegrate you.
[NEW] 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 leaves beast form. As usual, any leftover damage then applies to the druid's normal hit points. If the leftover damage leaves the druid with 0 hit points, the druid is disintegrated.
The problem, as I see it, is an
IF condition that generates two contrary effects. With the half-orc's Relentless Endurance it seems clear(er) that it won't protect you (because of the "and doesn't kill you outright" stipulation) while the issue of polymorph and wild shape is logically muddy.
The change to the wording of disintegrate seems to clarify polymorph and wild shape while giving a free boost to the half-orc. But the "reduces the target to 0 hit points" language has another problem. As pointed out in an answer to this question, it has the strange effect that "the spell would not disintegrate creatures that were already at 0 HP before being hit by the spell". This applies equally to disintegrate ray, death ray, falling stone blocks that crush creatures to a pulp, or any effect that has the wording "reduces the target to 0 hit points".
Furthermore—and this is not so much a rules question as a metarules issue—I find it irritating that the beholder's disintegration ray would work so differently from the disintegrate spell (since the wording of disintegration ray has not changed, so presumably the old Sage Advice ruling applies). While there nothing requiring logical consistency in a fantasy game, there are benefits (in terms of rule clarity and player sense of fairness) to having effects that are so similarly named operate in the same way.
So the question is whether there is a general rule that can govern interaction between these effects, or if it can only be resolved on a case-by-case basis.
* Not saying which one due to potential spoilers, but it is a WotC-published hardcover.