Ares (Guardians of Hades Series Book 1) by Felicity Heaton

Caged by the manifestation of his power over fire, Ares lives a cold existence driven by duty and the desire to return to the Underworld… until his world collides with a daemon who steals his power and a mortal female who shatters the ice around his heart and awakens the true fire within him—a soul-stirring passion both dangerous and seductive.