Energy Drained

The creature gains 1 or more negative levels. If the creature has at least as many negative levels as Hit Die, he dies. Each negative level gives a creature -1 penalty on attack rolls, saving throws, Skill checks, Ability checks, and effective level. (For determining the power, duration, DC, and other details of spells or special abilities.) Negative levels stack.

Energy Drained happens when Enervation or Energy Drain is cast on something living, or when a Wight, Wraith, or Spectre hits you.

The spell Restoration restores 1 negative level, as does using a Rest Shrine. The spell Greater Restoration restores all negative levels.