There's a special irony that comes when a hunter of the undead becomes afflicted by the very curse they seek to eradicate, and Xir is that contradiction made flesh. Inheriting some of her father's vampiric powers, she has taken up the family mantle; hunting down the undead, putting them to rest whether they like it or not.
Using the rituals of an ancient magical practice, she has made a pact with and incredibly powerful and otherworldly being named Mahathallah . Her familiar, Twink, mediates this pact, and imparts powers onto the young (by elven standards) witch. In exchange for her patronage, Mahathallah simply asked for a single favor. Xir knew full well the implications of owing a deific being undisclosed aid but took the deal, now plagued by dread of the day that debt is called in.
She's always watching, a shadow in Twink's eyes.