Twist the Knife

 
Prerequisite(s): Improved Critical (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.
 
Source: Exemplars of Evil