Set said:A race that is *almost* immortal and otherworldly, said to have been descended from fey, may well be looking for 'the way back' to the immortality, innate magical prowess and otherworldly / extraplanar nature they have lost. The realms of faerie are closed to them, and so they entreat with fiends, with their casters and warriors ending up in some transitive PrC that allows them to become native Outsiders at the highest levels (like Fiend-Blooded or Acolyte of the Skin or Monk or Favored Soul).
Hungry for the power that they feel in their blood, and which they feel they have had stolen from them by the fey races that abandoned them to the material plane, they bargain with demons to re-empower their blood and reject the weak failings of flesh. Why demons? Easier to trick and steal power from. Devils are too tricky...
May I suggest Shadar-Kai for that theme? Had very good experience with them, and they come complete with strange items and background. I made them humanoid (elf) in my campaign instead of fey, but thats up to you. They may be a bit close to Mystaras shadow elves though.