Lord of Shadows by Tanya Anne Crosby

Rhiannon’s destiny is fated from the moment of her birth. She and her dewine sisters must defeat an ancient evil, or Britain itself will go the way of Avalon. Her sisters and their champions have set the stage for a final, epic battle against the Witch Goddess Cerridwen, but Rhiannon can only fulfill her destiny if she can win the heart of the Lord of Shadows, the man who holds her prisoner.

Fueled by vengeance, Cael d’Lucy’s ambition has raised him to the pinnacle of power. Concealing an ancient secret, he’s close to achieving his desires, and once the dust of battle settles, he stands to rule again as Duke of Wales. All he must do is vanquish a slip of a girl—Morwen Pendragon’s daughter. Trouble is… Rhiannon is Cael’s match, mind, body and soul. Winning her means losing Blackwood; losing her means winning Wales, but what shall it profit a man if he gain the entire world, and lose his very soul?