Does a troll die if its maximum hit points is zero?

Question

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?

Answer 1

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):

... its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

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):

The troll dies only if it starts its turn with 0 hit points and doesn't regenerate

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.

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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:

Dropping to 0 Hit Points

When you drop to 0 hit points, you either die outright or fall unconscious, as explained in the following sections.

One of which is:

Falling Unconscious

If damage reduces you to 0 hit points and fails to kill you, you fall unconscious (see appendix A). This unconsciousness ends if you regain any hit points.

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):

Healing

... A creature's hit points can't exceed its hit point maximum, so any hit points regained in excess of this number are lost.

So the troll will remain unconscious at 0 HP.

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_{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:}

_{A monster usually dies or is destroyed when it drops to 0 hit points. For more on hit points, see the Player's Handbook.}

Answer 2

Trolls are a troll...

The Troll and Spirit Troll both possess the "Regeneration" trait which states as you mentioned:

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.

For the Spirit Troll, the "acid or fire" clause is replaced by "force or psychic".

The "Monsters and Death" rule which states:

Most DMs 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 DM might have them fall unconscious and follow the same rules as player characters.

Due to the "Regeneration" trait, both the Troll and Spirit Troll would be mighty villains who aren't instantly killed, instead, continually being brought back to 10 hit points on the start of their next turn after falling to 0 hit points, unless of course they took any damage of the type mentioned within the feature.
While the Troll is at 0 hit points, it will be unconscious, making it easy for players to apply the required damage type, but Spirit Trolls are immune to the unconscious condition, meaning they can cause more of a problem.

I mentioned in another post the conditions where Trolls can be instantly killed, or die from means not described so far:

Jeremy Crawford in the Dragon+ episode that aired 29th January 2019 that the instant death rule does not effect Trolls, Regeneration trumps that rule, so even reducing it to 0 hit points, with leftover damage equal to its max hit points, the Troll will still gain Regeneration buffs. However, while Regeneration trumps Instant Death, Disintegrate's chance of instant death trumps Regeneration, so you can still instant kill a Troll with that spell, as discussed in this post.

Further clarification is required that Disintegration is just one example of a spell that can trump the "Regeneration" trait, others may include Power Word Kill, Divine Word, or other spells that mention causing instant death.

What happens when a trolls maximum hit points has been reduced to zero?

This is actually much easier to answer than other Troll related questions as you can't heal hit points you don't have:

When a creature receives healing of any kind, hit points regained are added to its current hit points. A creature's hit points can't exceed its hit point maximum, so any hit points regained in excess of this number are lost.

Therefore the troll can't regenerate, and per the "Regeneration" trait's description "The troll dies only if it starts its turn with 0 hit points and doesn't regenerate." Even with JC's ruling on the general rule of Instant Death, the specific trait's description damns the Troll, and Spirit Troll to death.

Answer 3

The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.

The specific overrules the general.

The troll would regenerate to a max of 0hp then instantly become in a near-death state again. But it would not die.

When comparing the troll’s regeneration with ‘instant death’ abilities there is no way to resolve to conflict other than by DM ruling.

Answer 4

Yes

Regeneration: 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.

"Regaining" hit points means moving your current hp closer to your hp maximum. Regeneration can't bring a troll above its hp maximum - regardless of whether that maximum has been reduced. Therefore, a troll at max hp can't regenerate.

Meanwhile, Life Drain states that:

The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a Long Rest. The target dies if this Effect reduces its hit point maximum to 0.

A troll who has failed its Constitution save against a wraith's Life Drain will not be able to regenerate past its new, lowered max hp. If that max hp is 0, according to the effects of Life Drain, the troll would die. And since regenerate can't function if the troll is at max health, it doesn't stop the troll from dying.