Children of Ash by Allison Anderson

Axel Ashersson is the greatest hero in Åldran song. As the only son ever born to a Häxa, women gifted by the gods with magic, he is unmatched in strength and impervious to mortal injury. But with his power also comes a curse. Prophesied when he was but a babe, if Axel ever trades his heart to another, he will lose his gifts and Åldras will fall in the war against the Sidan invaders. Luckily, only one woman has ever had any claim to his heart and he's under no illusions that she'll ever give her heart to him.