The book of Azrael

For a thousand years, the Etherworld has known peace. Until now. Many centuries ago, desperate to save her dying sister, Dianna made a deal with Kaden, a monster far worse than any nightmare. Locked in servitude to him, she is forced to hunt down an ancient relic held by her most dangerous enemies: an army led by Samkiel, the World Ender. After the Gods War, Samkiel hid from everything, denying his crown and deserting his people. Now, an attack on those he loves sends him back to the realm he never wished to return to, and into the sights of an enemy he had hoped to forget. With every world at stake, Dianna and Samkiel are forced to set aside their animosity and work together, before all is lost...