Death Devotion( Complete Champion, p. 57)
The power of death imbues your weapon with exceptional might.
Once per day as a swift action, you can cause one of your melee weapons to radiate negative energy for 1 minute. When you make a successful attack with this weapon, the target must succeed on a Fortitude save (DC 10 +1/2 your character level + your Cha modifier) or gain a negative level. You can bestow only one negative level per target for every four character levels you possess (minimum one, maximum five negative levels at 20th level).
You can select this feat multiple times, gaining one additional daily use each time you take it.|||If you have the ability to turn or rebuke undead, you gain one additional daily use of this feat for each three daily turn or rebuke uses you expend.