The Troll has the Regeneration feature which states:
The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.
I'm wondering what happens when even a trolls maximum hit points has been reduced to zero, say by the Wraith's Life Drain feature. I don't know if this method works to kill a troll because I'm unsure whether they have actually regenerated or not. Does the troll die?
Best Answer
It comes down to whichever a DM decides is more specific
We know that specific beats general, but the trouble is knowing which rule is more specific. We know that the standard damage rules are more generic than both the troll's Regenerate feature and, say, a wraith's Life Drain action.
From a wraith's Life Drain action (MM, p. 302):
So if a gang of wraiths jumped a troll and managed to get its maximum HP down to 0, then if a DM decides that the above quote is more specific than (from MM, p. 291):
then the troll dies, otherwise, the troll does not. Even JC has been inconsistent with his rulings on the matter, so it really is up to the DM to decide.
However, if a DM rules that the troll does not die, it likely can't do anything until its maximum HP is restored. (Note that the following is based on rules for players1).
From PHB, p. 197:
One of which is:
If the troll does not die outright, then they are unconscious. Since they cannot actually regain hit points, regardless of their Regenerate feature, since (PHB, p. 197):
So the troll will remain unconscious at 0 HP.
1. I know that monsters don't necessarily use the same rules as players, but as far as I am aware, there are no monster rules for what happens at 0 maximum HP. In fact, what little there is on monster HP from the MM, p. 7, implies that the rules I cite are relevant after all: