SRD:Energy Drain, Negative Levels, and Level Loss

Energy Drain (Su)
This 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.

Negative Levels
An affected opponent takes a –1 penalty on all skill checks and ability checks, attack rolls, and saving throws, and loses 5 hit points and 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. If she has more than one spell at her highest level, she chooses which she loses. A psionic character loses access to one power per negative level from the highest level of power he can manifest; he also loses a number of power points equal to the cost of that power. If two or more powers fit these criteria, the manifester decides which one becomes inaccessible. 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.

A character with negative levels at least equal to her current level, or drained below 1st level, is instantly slain. Depending on the creature that killed her, she may rise the next night as a monster of that kind. If not, she rises as a wight.

Level Loss
A character who loses a level instantly loses one Hit Die. The character’s base attack bonus, base saving throw bonuses, and special class abilities are now reduced to the new, lower level. Likewise, the character loses any ability score gain, skill ranks, and any feat associated with the level (if applicable). If the exact ability score or skill ranks increased from a level now lost is unknown (or the player has forgotten), lose 1 point from the highest ability score or ranks from the highest-ranked skills. If a familiar or companion creature has abilities tied to a character who has lost a level, the creature’s abilities are adjusted to fit the character’s new level.

The victim’s experience point total is immediately set to the midpoint of the previous level.