Heart of the Nabassu( Fiendish Codex I: Hordes of the Abyss, p. 86)
Your ancestry traces back to a place where the Abyss meets the Negative Energy Plane.
You can absorb harmlessly a number of negative levels per day equal to the number of Abyssal heritor feats you possess. For example, if you have two Abyssal heritor feats, you can gain up to two negative levels on a given day and not suffer any ill effects.
Your barest tie to the Negative Energy Plane is offputting to animals. You take a -2 penalty on Handle Animal checks.