The Forsaken (Book 3: The Guardian Series):

The war with Odette has begun. Cities around the world have fallen to her careful manipulations, and the world as Clare knows it has ended in blood. Worse still is the threat of a darker evil that lingers over Clare as she searches for a way to end the war before anyone else dies. Will she be enough to face the growing threat? Can she stop Odette before it is too late? 'The Forsaken' is the third book in The Guardian Series.