Dark Invocation; 8th; Eldritch Essence
This eldritch essence invocation allows you to change your eldritch blast into an utterdark blast. An utterdark blast deals negative energy damage, which heals undead creatures instead of damaging them (much like inflict spells). Any creature struck by the attack must make a Fortitude save or gain two negative levels.
The negative levels fade after 1 hour. If a target ever has as many negative levels as Hit Dice, it dies.
Source: Complete Arcane