The creature gains 1 or more negative levels. Each negative level gives a creature a -2 penalty on attack rolls, saves, skill checks, and ability checks. Also inflicts a -1 penalty to effective level (affects things like spells and abilities which rely on caster level or character level). Negative levels stack.
If the target has as many negative levels as its Hit Die or levels, it dies.
The spells Restoration and Restoration, Mass restore 1 negative level, as does using a Rest Shrine or waiting 1 minute in a tavern. Cleric's Radiant Servant: Positive Energy Burst ability cures 1d4 negative levels. The spell Greater Restoration restores all negative levels.
Negative levels regenerate at a rate of 1 level every 2 minutes.