D&Dtools1

The world around you takes on a dim and muted tone as you are encased in a tight-fitting sheath of crackling negative energy.
This spell bestows a negative level upon any non-undead creature that makes a successful melee attack against you (regardless of whether the attack deals damage).
There is no save, but spell resistance applies.
Retributive enervation can bestow a number of negative levels equal to 1/2 your caster level (maximum 10).
Once this limit is reached, the spell ends, even if the duration has not yet expired.
Assuming a creature that gains negative levels survives, it regains lost levels after a number of hours equal to your caster level (maximum 20 hours).
Usually, negative levels have a chance of permanently draining the victim's levels, but the negative levels from retributive enervation don't last long enough to do so.
An undead creature that makes a successful melee attack against you while this spell is active gains 5 temporary hit points that last for up to 1 hour.