How does the troll's Regeneration trait work?

Question

The Troll stat block includes the following trait:

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.

(The same stat block appears for Frost Giant Everlasting Ones in Volo's Guide to Monsters (p. 148), and likely for some other monsters too.)

My reading of the trait is that, unless the monster took acid or fire damage on its last turn, it will be guaranteed to benefit from regeneration at the start of its turn. If that's true, the only way to effectively kill the troll (in this case, or others with this trait) is to use fire or acid damage, making the monster pretty tough to kill.

Am I reading that correctly? Or is there a different intended interpretation that I'm missing?

Answer 1

You are reading this correctly. Trolls are notoriously difficult to kill, able to regenerate back from pretty much anything (including things like decapitation) in minutes.

This also becomes pretty clear if you read their lore block, and adventurers do well to prepare if they are going up against these beasts.

Keep in mind though, that a single point of fire or acid damage is enough to disable regeneration, so smashing the Troll down to 0 hp and then burning it with Alchemist's Fire or Acid will make sure it can't recover. Alternatively, dragging it into a campfire also works.

Once the Troll is disabled and there are no other monsters left, it's usually quite doable to deal with one.