Mortal Heart — Robin LaFevers

In the powerful conclusion to Robin LaFever's New York Times bestselling His Fair Assassins trilogy, Annith has watched her gifted sisters at the convent come and go, carrying out their dark dealings in the name of St. Mortain, patiently awaiting her own turn to serve Death. But her worst fears are realized when she discovers she is being groomed by the abbess as a Seeress, to be forever sequestered in the rock and stone womb of the convent. Feeling sorely betrayed, Annith decides to strike out on her own.

She has spent her whole life training to be an assassin. Just because the convent has changed its mind, doesn't mean she has.