Maiming Strike( Exemplars of Evil, p. 25)
You can make dreadful attacks that disfigure your opponent.
Sneak attack +2d6, Evil,
Whenever you make a successful sneak attack on a target creature, you can reduce your extra damage and deal Charisma damage instead. For every 2 dice of extra damage that you sacrifice, your attack deals 1 point of Charisma damage.