Darkest. The Possession Series, #2

As darkness devours the earth, loyalty becomes the sharpest blade. Bound to a demon and haunted by the woman he lost, Tristan Gao will stop at nothing to reclaim Sarrafin's soul. But every step toward her draws him closer to the darkness stirring inside, and to the ancient prophecy that could unmake creation itself. With his angelic Guardian, Vxatghuh, at his side, Tristan crosses continents and descends into realms of forbidden power, seeking a way to save Sarrafin without freeing the Demon King.
Yet as new allies rise and old sins awaken, the line between salvation and damnation begins to fade.