Gloom Strike( Drow of the Underdark, p. 54)
Your sneak attacks leave a residue of darkness that clouds your enemy's vision.
Sneak attack +3d6, darkness as a spell-like ability,
Your sneak attack creates an aura of magical darkness that impedes your target's vision for 3 rounds. The target takes a —4 penalty on Spot checks and treats all other creatures as having concealment. Neither darkvision nor low-light vision pierces this effect, though the ability to see through magical darkness does. Any spell with the light descriptor suppresses the effect.Using this feat reduces your sneak attack damage by 2d6.