ToME: the Tales of Maj'Eyal

Four times now, I've had monsters that simply refuse to die. They are at or near 0 hp, they do not act in any way, and further damage to them "kills" them as many times as appropriate, but they do not disappear.

As far as I can tell, they do not give xp when re-killed, nor do they restore a corruptor's vim, but Drain does work on them for infinite replenishment. They also count towards counter-based achievements.

Oh, and they heal at their natural rate, though this does not change their timeless status.

I've only seen this once. The only thing that "fixed" the enemy for me was to use time skip on the enemy (as a Paradox Mage or Temporal Warden, though anyone could do it with Rune of the Rift). Once the enemy returns to reality from wherever enemies go when you banish them, I was able to immediately kill it next turn.