According to Fiend Folio, there are apparently certain rituals that can be performed to gain the benefits of a Fiendish Graft.  Antimony's certainly been delving into her heritage, so it wouldn't be out of the question for her to have come across and conducted one such ritual to get the wings of an Erinyes.<br/><br/>And having the wings now would allow me to replace Fly on her spell list with a spell more thematically appropriate for a Drow, such as Spiderskin...