I'm kinda late to the party, but in Baldur's Gate 2, every vampiric creature can drain your levels; get to level zero and you have no hit points left, so you die (and can't be resurrected either).
What's more annoying is that the effect is permanent until removed, the recovery spell is a high level one and can be dangerous to cast in the middle of a dungeon (automatically tires out the caster), and you stop getting XP while level drained.