Goddess by Josephine Angelini

The third and final book in the internationally bestselling Starcrossed series
She must rise, or they will fall...
Helen's powers are increasing - and so is the distance between her and her mortal friends. To make matters worse, the Oracle reveals that a dangerous traitor is lurking among them, and all fingers point to Orion. Still unsure whether she loves him or Lucas, Helen is forced to make a terrifying decision, or risk all-out war.