Thrall to Demon( Book of Vile Darkness, p. 50)
The character formally supplicates himself to a demon prince. In return for his obedience, the character gains a small measure of power.
Once per day, while performing an evil act, the character may call upon his demonic patron to add a +1 luck bonus on any one roll.
Once a character takes this feat, he may not take it again; he may not be the thrall of more than one demon. Nor may he take the Disciple of Darkness feat.