Energy Drain (Su)
This attack saps a living opponent’s vital energy. With each successful melee attack, the creature bestows one or more negative levels. If an attack that includes an energy drain scores a critical hit, it drains double the given amount. For each negative level inflicted on an opponent, the draining creature heals 5 points of damage. If the amount of healing is more than the damage the creature has taken, it gains any excess as temporary hit points that remain for a maximum of 1 hour.
For each negative level, the opponent takes a –1 penalty on all skill checks and ability checks, attack rolls, and saving throws, and loses one effective level or Hit Die (whenever level is used in a die roll or calculation). A character with spellcasting ability loses the ability to cast one spell of the highest level he or she can cast (player’s choice); this loss persists until the negative level is removed.
Negative levels remain until 24 hours have passed or until removed with a spell. If a negative level is not removed before 24 hours have passed, the afflicted opponent must attempt a Fortitude save with a DC of 10 + 1/2 draining creature’s HD + draining creature’s Charisma modifier (the exact DC is given in the creature’s Species Traits). On a success, the negative level goes away with no harm to the creature. On a failure, the negative level goes away, but the creature’s level is reduced by one. A separate saving throw is required for each negative level. A creature that loses all of its levels or Hit Dice dies and, depending on the source of the energy drain, might rise as an undead creature of some kind.