Soul Drain (3.5e Variant Rule)
From D&D Wiki
This variant uses the Magic of Incarnum supplement rules.
As a full-round action, you can attempt to deliver a soul drain to a helpless opponent. A soul drain is a melee touch attack but the target must not consider you a threat.
The defender must make a Will save (DC 10 + Your willsave modifier) or die. Essentia bonuses to Will saves do not apply to the DC. If the creature dies you gain +1 bonus essentia semi-permanently, any effect that restores the target to life removes this bonus essentia.
In all ways not listed here, a Soul drain behaves like a coup de grace. A character who can perform a Soul drain cannot perform a Coup de Grace, but may benefit from feats or abilities that improve Coup de grace(though these benefits apply to Soul Drain is possible).