The Monsters We Are by Suzanne Wright

The Ancients and the Aeons. Two groups of powerful immortals on the brink of destruction, and at the heart of it all stands Wynter Dellavale.

Wynter is a witch who once sought sanctuary for herself and her coven in Devil’s Cradle. Better known as ‘the Home of Monsters’, it was a place for outcasts and fugitives. She now wears the brand of Cain, the infamous Ancient, who has claimed her soul for all of eternity and doesn’t plan on ever letting go.
Together, Wynter and Cain have prevailed over many powerful enemies, but now they face their final – and most terrifying – battle. As the last Aeon left standing, the all-powerful Adam is out for revenge. His terms: Cain and Wynter in exchange for peace. If Devil’s Cradle won’t deliver them, the full force of Adam’s devastating rage will fall upon them all. As the stakes are raised, Cain and Wynter will do whatever it takes to defeat Adam and keep each other safe, even if it means unleashing the monsters that live within them . . .