The Bone Flower Throne by TL Morganfield

Tenth century Mexico is a world in great upheaval. Incursions from the north bring constant warfare as clashing politics and faiths battle for supremacy. At the fore stands Smoking Mirror, a bloodthirsty god bent on subjugating humanity with his hunger for human hearts. Now his high priest, Ihuitimal, has conquered Culhuacan, one of the Toltecs' most powerful kingdoms.

When Smoking Mirror's archenemy, the Feathered Serpent, tasks Quetzalpetlatl, exiled princess of Culhuacan, to help his mortal son Topiltzin end human sacrifice, she eagerly embraces her new destiny. It means avenging her father's murder at the hand of her uncle Ihuitimal, but for Topiltzin-who's more than just a brother to her-she will do anything. Even sacrifice her own heart.