The Devil's Bargain — Judith Tarr

National bestselling author of Pride of Kings and World Fantasy Award nominee conjures a fascinating world of sorcerers and warriors in this dramatically powerful tale.  

Led by Richard the Lionheart, king of the English and count of Anjou, the armies of the West are conducting a crusade against the sultan Saladin. In Cyprus, the king's mother, Eleanor, has struck a devil's bargain with the sorcerer Sinan, the Old Man of the Mountain, to grant Richard victory at any cost. Only Sinan knows that victory will cost the king his immortal soul.
 
Sioned, an illegitimate child of a princess of the Celts and the late king of England, is heir to twofold magic: the faery Sight of ancient Wales and the demonic powers of the Devil's brood of Anjou. Summoned by her brother Richard to save both the Crusade and his soul, Sioned receives help from two unlikely allies -- a prince of the infidels and an army of the jinn -- as she races against time and the powers of the darkness. But with the extent of her abilities still untested and only her instincts to guide her, will she be able to swing the sorcerous struggle back toward the powers of the light?