Vespertine

From the New York Times bestselling author of Sorcery of Thorns and An Enchantment of Ravens comes a thrilling new YA fantasy about a teen girl with mythic abilities who must defend her world against restless spirits of the dead.
The dead of Loraille do not rest.
Artemisia is training to be a Gray Sister, a nun who cleanses the bodies of the deceased so that their souls can pass on; otherwise, they will rise as twisted spirits with a ravenous hunger for the living. She'd rather deal with the dead than the living, who point and whisper about the odd girl who was once possessed by a violent spirit.
When her convent is attacked by possessed soldiers, Artemisia fights back by awakening an ancient spirit bound to a high saint's relic. It is a revenant, a malevolent being now whispering in her head. Wielding its extraordinary power almost consumes her in body and soul. But death has come to Loraille, and only a vespertine, a priestess...