Dark Desires After Dusk ~ Kresley Cole

this is the sixth book in the Immortals series.

Cadeon the kingmaker has found his mate, unfortunately, demons are forbidden from taking mates from mortals, as they can’t survive the bonding with demons. Suddenly, Cadeon is told that he has to take the Vessel to Groot the Metalurgist, and exchange her for a sword that will kill the usurper who has his brother’s throne. Problem is, the Vessel is his mate. Can Cade really resist claiming his mate, and then turn her over to Groot, in order to restore his brother’s crown?

Holly is a mathmatics student, working on her PhD. and teaching math to jocks at Tulane. Suddenly she is drugged and captured, when she comes to, she is chained naked to an alter with several demons about to assult her. In her rage at the situation, she shrieks and calls down lightning, which kills the demons surrounding her, and begins her transition into an immortal Valkarie. But does she have to stay this way?