Vespertine

Artemisia is training to be a Gray Sister, tasked with cleansing the bodies of the dead so that their souls can pass on; otherwise, they will rise as spirits with a hunger for the living.
When her convent is attacked, Artemisia defends it by awakening a revenant. Wielding this ancient spirit's extraordinary power almost consumes her - but only a vespertine, a priestess trained to wield a revenant, has any chance of saving Loraille.

As Artemisia unravels a sinister mystery of saints, secrets and dark magic, her bond with the revenant grows. And when a hidden evil begins to surface, she discovers that facing this enemy may require her to betray everything she's been taught to believe - if the revenant doesn't betray her first.