Part of the Troll's Regeneration feature says:
The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.
Does this case beat instant kill effects, such as Divine Word or Power Word Kill?
Divine Word says:
[…] On a failed save, a creature suffers an effect based on its
current hit points.[…]
- 20 hit points or fewer: killed instantly
And Power Word Kill says:
If the creature you chose has 100 hit points or fewer, it dies. Otherwise, the spell has no effect.
In addition to such spells, there are more general effects that cause death, mainly massive damage and failing three death saves (if the DM actually rolls death saves for the troll).
It seems like the Regeneration trait enumerates the only case where a Troll can "die". But is Regeneration specific enough that it beats all other death effects?
I am looking for a RAW interpretation. Were I running a game and someone Power Word Killed a troll, I would have it die – but I'm not convinced that outcome is RAW.
Best Answer
The rules as written are ambiguous - and so it's up to the DM's ruling (and designer clarification has gone both ways)
Unfortunately, it's not clear exactly how these rules interact. The troll has an ability which apparently specifies the only circumstances in which a troll dies, and these spells can cause creatures to die instantly; both are exceptions to the normal rules about how things can die, so the principle that "specific beats general" guides us in trying to resolve the conflict, but judging which of the two features is more specific seems subjective and ambiguous.
If you interpret the spell's rules as being more specific than troll regeneration, the troll dies; and since it is dead, regeneration becomes meaningless. If you interpret the troll's regeneration as being more specific than the spell, it precludes the death from happening despite the spell's effect.
Purely RAW ruling, on specific-beats-general principles, I would personally read the troll's ability as more specific than the spell; the spells can, after all, be used on many different kinds of creatures by many different casters, but a troll's regeneration is only ever about trolls, so it is necessarily much more limited in scope and therefore takes precedence over the rules of the spells.
JC says the troll dies
Official D&D 5e rules guru Jeremy Crawford weighed in when Rubiksmoose asked him about this issue on twitter:
In this case he's just repeated previously given clarification that effects which state creatures are killed or die do not function by reducing the target's HP to 0, they just kill the target directly. This fails to address the actual cause of the ambiguity, so we can't really take it as a clarification of the rules as written.
The response in context implies that Crawford believed that this aspect of the troll's regeneration ability is only meant to stop the troll dying due to normal hit point damage (as a "feature that relies on you having 0 hit points"), not prevent any other effect which reasonably causes death.
And then JC says the troll doesn't die
When asked a very similar question on a later podcast, about whether or not the instant death (massive damage) rule could kill a troll, he suggested that it should not:
Instant death by massive damage doesn't kill you by dropping your hit points to zero; it just says you die. Dropping your HP to zero is a necessary precondition for this rule to apply, but the statement that "you die" isn't any more qualified than the effect of Power Word Kill. Coming back to it later, JC seems to have taken a more literal reading of the troll's regeneration ability and ruled that it really does only die if it meets the requirements specified by its regeneration ability.
It seems plausible that if he'd been asked specifically about trolls and Power Word Kill again, he might have ruled differently, depending on whether he thought the spell was more specific than the troll's trait. He does preface the judgement by explaining how loathe he is to make rulings about general hypotheticals, because D&D isn't a coherent rules system, and would probably argue if challenged that this ruling was specifically about instant death and trolls, whereas the previous was about Power Word Kill/Disintegrate and trolls, so they don't conflict at all.