Twist the Knife( Exemplars of Evil)
You can make a vicious attack that leaves your victim gasp- ing in pain.
Improved Critical (PH) (melee weapon) , Sneak attack +2d6,
Benefit: Whenever you confirm a critical hit while wielding a melee weapon for which you have the Improved Critical feat, you can forego the extra damage and wrack the target with pain instead. For a number of rounds equal to the weapon's critical multiplier, the target takes a —3 penalty on attack rolls, weapon damage rolls, saving throws, ability checks, and skill checks.