The Darkest Surrender (Lords of the Underworld #8) by Gena Showalter

Possessed by the demon of Defeat, Strider cannot lose a challenge without suffering unimaginable pain. For him, nothing stands in the way of victory. Until Kaia, an enchanting Harpy, tempts him to the razor's edge of surrender.

Known among her people as The Disappointment, Kaia must bring home the gold in the Harpy Games or die. Strider is a distraction she can't afford because he has an agenda of his own-steal first prize, an ancient godly artifact, before the winner can be named. But as the competition heats up, only one prize will matter-the love neither had thought possible...