This supernatural attack saps a living opponent's vital energy and happens automatically when a melee or ranged attack hits. Each successful energy drain bestows one or more negative levels (the creature's description specifies how many). If an attack that includes an energy drain scores a critical hit, it drains twice the given amount. Unless otherwise specified in the creature's description, a draining creature gains 5 temporary hit points (10 on a critical hit) for each negative level it bestows on an opponent. These temporary hit points last for a maximum of 1 hour.
An affected 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) for each negative level. A spellcaster loses one spell slot of the highest level of spells she can cast and (if applicable) one prepared spell of that level; this loss persists until the negative level is removed.
Negative levels remain until 24 hours have passed or until they are removed with a spell, such as restoration. If a negative level is not removed before 24 hours have passed, the affected creature must attempt a Fortitude save (DC 10 + 1/2 draining creature's racial HD + draining creature's Cha modifier; the exact DC is given in the creature's descriptive text). 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 also reduced by one. A separate saving throw is required for each negative level.