The Book of Azrael by Amber V. Nicole — book slip

A thousand years ago, Dianna made a desperate bargain to save her sister, binding herself to Kaden, a monster of nightmares. Now, she must retrieve an ancient relic from the very creatures that hunt her.

Meanwhile, Liam, once known as Samkiel the World Ender, emerges from self-imposed exile after an attack on his people. As enemies older than time, Dianna and Liam must set aside their differences to save their worlds from destruction.

For fans of Sarah J. Maas and Kerri Maniscalco, this is the first book in the 'Gods & Monsters' series.