ENERGY DRAIN AND NEGATIVE LEVELS
Some horrible creatures, especially undead monsters, possess a fearsome supernatural ability to drain levels from those they strike in combat. The creature making an energy drain attack draws a portion of its victim's life force from her. Most energy drain attacks require a successful melee attack roll
-mere physical contact is not enough. Each successful energy drain attack bestows one or more negative levels on the opponent. A creature takes the following penalties for each negative level it has gained.
-1 effective level (whenever the creature's level is used in a die roll or calculation, reduce it by one for each negative level).
If the victim casts spells, she loses access to one spell as if she had cast her highest-level, currently available spell. (If she has more than one spell at her highest level, she chooses which she loses.) In addition, when she next prepares spells or regains spell slots, she gets one less spell slot at her highest spell level.
Negative levels remain for 24 hours or until removed with a spell, such as restoration.
After 24 hours, the afflicted creature must attempt a Fortitude save (DC 10 + 1/2 attacker's HD + attacker's Cha modifier). (The DC is provided in the attacker's description.) If the saving throw
succeeds, the negative level goes away with no harm to the creature. The afflicted creature makes a separate saving throw
for each negative level it has gained. If the save fails, the negative level goes away, but the creature's level is also reduced by one.
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. A creature gains 5 temporary hit points
for each negative level it bestows (though not if the negative level is caused by a spell or similar effect).