Demonsworn Knight( Champions of Ruin, p. 22)
A scornful champion of the demon princes, you detest and oppose devils and other creatures that refuse to heed the call of chaos.
Thrall to Demon (CoR) , base attack bonus +4,
Any weapon you wield is treated as chaotic- and evil-aligned for the purpose of overcoming damage reduction. In addition, against lawful outsiders, you deal an extra 1d6 points of damage with any successful weapon attack.
This feat can only be taken once. A character who takes this feat cannot also take the Hellbound Knight or Tormented Knight feat. A fighter can select this feat as one of his fi ghter bonus feats. In addition, your eyes turn dead black, and your teeth become as sharp as fangs.