Looking for ideas: mortally wounded

[FrancisBean]

I'm looking for a way to spice up a few boss monsters in my campaign. I want the idea that, even when mortally wounded -- that is, down to 0 HP -- they're still dangerous. Picture the dragon so large that even when you've pierced it through the heart, it still has a few rounds of fight left in it before the rest of the monster knows it's dead. Has anybody played with this concept?

It seems to me that it's only a good idea when dealing with large+ creatures with inhuman stamina, and it completely breaks once we add healing. At least, unless we also have a house rule which adds negative HP and makes healing account for them. I'm not looking to go that far.

[JackPhoenix]

How is it different from simply giving them extra HP?

[Kane0]

Each round past 0 they gain 1d6 levels of exhaustion and cannot be healed.

[Man_Over_Game]

How is it different from simply giving them extra HP?

Because the goal then isn't to deal damage.

Core of a hellfire elemental is about to explode. Same effect as a Delayed Fireball, but nullified on Cold damage.

Eldritch horror loses its magical resistance but continues to thrash about. It devours itself if it cannot deal damage to another creature before the start of its next turn.

The boss's soul is still intact through its connection to the Astral Plane. It's ghost is immune to nonmagical or necrotic damage, and it's resistant to damage that isn't Radiant, Slashing, Piercing or Bludgeoning. Dealing 20 damage to the ghost, or 100 damage to its corpse, severs the ghost's astral connection to its body, and the ghost disappates for good.

[JNAProductions]

Each round past 0 they gain 1d6 levels of exhaustion and cannot be healed.

This is a good general solution.

MOG also has good ideas for more specific ones.