Level drain, or “energy drain”, is when a level of experience is removed from a character.
The hit points rolled for the drained level are temporarily removed from the character's hit points.1) The player should not destroy the record of the lost hit die roll however, since it will be used when the level is regained. Hit dice are never re-rolled in cases of level drain.2)
The highest class level is drained. If there is a tie, the class with the most XP will be drained. Further ties are resolved by a die roll.
The most recently gained level is drained.