There are a number of questions on this website and online about the interaction between effects that cause death at 0 hit points and functions that prevent death at 0 hit points.
Effects that result in death if you reduce a character to 0 hit points include:
- Disintegrate Spell (Original): "If this damage reduces the target to 0 hit points, it will be resolved."
- Disintegrate Spell (PH Errata 2.0): "The target will be resolved if this damage leaves 0 hit points."
- Viewer Decay beam: "If this damage reduces the target to 0 hit points, its body becomes a pile of fine gray dust."
- Viewer Death ray: "The target dies if the beam reduces it to 0 hit points."
- A published adventure * contains a large number of effects with the language "If this damage reduces the creature's hit points to 0, it will be crushed to pulp."
- Immediate death: "If the damage reduces you to 0 hit points and there is still damage left, you die if the remaining damage equals or exceeds your maximum point maximum."
The effects that can prevent a character's death if a character is reduced to 0 hit points include (HT on this question for some of them):
- Halborcs Relentless perseverance: "If you were reduced to 0 hit points but were not killed immediately, you can drop to 1 hit point instead."
- Barbarian Relentless anger: "If you drop to 0 hit points during the rage and do not die immediately, you can make a constitutional roll of DC 10. If you are successful, you drop to 1 hit point instead."
- Druid Wild shape: "You automatically return when you pass out, drop to 0 hit points, or die. (…) If you return to your normal form, you will return to the number of hit points you had before your transformation."
- Death station Spell: "If the target first falls to 0 hit points due to damage, the target drops to 1 hit point instead and the spell ends."
- Polymorph Spell: "The transformation lasts for the duration or until the target falls to 0 hit points or dies. (…) When it returns to its normal form, the creature returns to the number of hit points it had before the transformation."
The interaction between these effects was an ongoing question. Sage Advice previously made the following decision:
If the damage from disintegrate reduces a halborc to
0 hit points, relentless endurance can prevent the orc
transform from ashes? If disintegrate reduces you to 0 hits
Points, you will be killed immediately if you turn to dust. If you have one
Halborc, Relentless Endurance cannot save you.
What happens if a druid using Wild Shape is reduced to?
0 hit points from disintegrate? Does the druid just walk?
Animal shape? The druid has turned to dust since the spell disintegrates The moment you drop to 0 hit points.
But then the wording of the disintegrate The spell was changed and in Sage Advice 2.0 the following decision appeared:
(NEW) If the damage from disintegrate reduced half
Orc at 0 hit points, relentless endurance can prevent that
Transform Orc of Ashes? Yes. The disintegrate Spell turns
Turn them into dust only if the damage done by the spell leaves you with 0 hits
Points. If you're a Halborc, Relentless Endurance can spin
The 0 into a 1 before the spell can dissolve you.
(NEW) What happens when a druid uses wild shape?
reduced to 0 hit points by decay? Does the druid
Just leave animal form? The druid leaves an animal shape. As
Normally, the remaining damage will apply to the druid's normal hit points. If the remaining damage leaves the druid with
0 hit points, the druid is dissolved.
The problem is, as I see it, a
IF Condition that creates two opposite effects. With the Halborcs Relentless perseverance it seems clear (um) that it habit Protect yourself (because of the condition "and don't kill yourself immediately") while the subject polymorphic and wild shape is logically mud.
The change of wording from disintegrate seems to clarify polymorphic and wild shape while giving the Halborc a free boost. However, the language "reduces the target to 0 hit points" has a different problem. As stated in an answer to this question, it has the strange effect that "the spell would not Decay of creatures that already had 0 HP before being hit by the spell ". This also applies to Beam decay, Death rayfalling blocks of stone that crush creatures into porridge, or an effect that "reduces the target to 0 hit points".
Furthermore – and this is less a rule question than a metarule question – I find it irritating that the viewer Decay beam would work so differently from that disintegrate spell (assuming that the old answer from Sage Advice applies because the wording of has Decay beam has not changed). While there's nothing in a fantasy game that requires logical consistency, there are advantages (in terms of player clarity and fairness) when effects that use virtually the same name work the same way.
So the question is whether there is one General Rule that can regulate the interaction between these effects or that can only be solved on a case-by-case basis.
* Don't tell which one because of possible spoilers, but it's a WotC-published hardcover.