Rite of the Omega by Eva Dresden

Orphan. Jealously guarded possession. The object of her cruel guardian’s desires.

These are the only things Aida Vertia has known in her life. Locked away in the high towers of Logoria, the seat of Aeslomor, she has little contact with the outside world. Knowing nothing of its splendor and beauty, she sees only the cruelties within the sooty stone walls of her prison.
Then he comes on the crest of a raging inferno. Destroying everything she’s ever known. Freeing her from one prison for the cold, hard chains of the next.
They call him the Usurper. A blood crazed mage, no different than the one before.
Yet he is wholly different.
Forcing her to bend to his will, she is helpless to stop the way he makes her soul sing. No matter the murderous vengeance in his eyes or the vicious oaths he swears.
He claims she has a purpose that he’ll see fulfilled, but amid the ruins of her past, she’ll find a destiny far more devastating.