Under an Onyx Sky (Elemental Enchanters Series Book 4) by Richards, Carrigan

This is the end.Now that Ava and the rest of the Elemental Enchanters have allied with the Cimmerians, they must prove their allegiance by doing unthinkable tasks which test each one of them.As Ava becomes close to the Cimmerians, friendships develop, and she finally learns the horrifying truth behind the war against the Ephemerals.Now, with her own motives in question, she must decide whether or not to end the war.