Oath of the Alpha by Eva Dresden

Captive. Sacrifice. Ultimate power.

This is Aida’s life now. Among the ruins of her birthright, the Usurper demands the incredible power she harbors for his own. King Er’it will make of her a bloody sacrifice, vowing to steal the magnificent force of her magic.
Yet they are both torn between bitter hatred and savage need as Aida learns what it means to be free and Er’it swears his oaths. An irresistible bond pulls them closer, growing stronger day by day as they near the ultimate end of their journey.
The great King is as helpless as his captive Omega to deny the claim they set upon each other.
In the end, they must discover if the need for power will succeed or if the heart will triumph.