I think she's going to be a hexblade warlock. I'd like to play this as a patron who actually demands stuff in return for the power it bestows. Kind of alternating between her using the power to pursue her personal agenda, then she gets a quest in a dream / vision / etc. and has to complete it to retain access to the arcane power.

The hexblade patron is supposed to be the Raven Queen, who is a dark figure, but is very strongly hateful of real undead and those necromancers that play with the natural cycle. Also, she is associated with dreams and visions, and [i]collects[/i] them in some way. I don't remember why, but I have a vague memory of reading this.