To Break a Dark Cage by Kathryn Ann Kingsley

He was kissing her with a fury that threatened to devour her. She needed this. Needed the wrath and the desire. One last time.
Gwen is a prisoner of the man she loves…
In the battle for Avalon, she sided with Mordred’s enemies and allowed the demon Grinn to grow more powerful than ever.
Passion still burns fiercely between Mordred and Gwen. But the dark prince crushes his emotions and tells Gwen that he is sending her home. That she will never see Avalon—or him—again.
But Avalon has ideas of its own. The magic of the island draws Gwen back and gifts her with stunning new abilities.
Gwen has one last chance to save the island from a devastating war that would destroy everything—and everyone—she has come to love. But can she tame Mordred’s rage and pride? And can a demon ever be trusted?