Needs review
Source: Exemplars of Evil

Description: You can make a vicious attack that leaves your victim gasping in pain.

Prerequisites: Improved Critical, Sneak Attack +2d6

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.