Vile Traitor's Blade (3.5e Feat)
From D&D Wiki
Vile Traitor's Blade [Vile]
Your sneak attacks gain the evil descriptor and weaken good-aligned foes.
When you attack an opponent that is denied his Dexterity bonus to AC, this attack is treated as if made with an evil-aligned weapon (i.e. for overcoming damage reduction). Against good-aligned enemies, your sneak attacks also deal 2 points of Constitution damage.