Her Soul for Revenge by Harley Laroux

Juniper

After a cult tries to sacrifice me to their wicked god, I went on the run, doing whatever was necessary to survive. Until a demon offered me a deal: give him my soul and you’ll help me claim the vengeance I seek. Blood will be spilled, and the monsters I once ran from will soon be running from me. But damning my soul was just the beginning— It’s my heart the demon wants next.

Zane

I’ve been hunting souls for centuries, but she is the ultimate prize— vicious and feral with a broken soul dark as my own.  I thought claiming her would be a simple game, but Juniper is far from simple. I chose to follow her on a path drenched with the blood of her enemies, but it’s our blood that may be spilled next. As an ancient god from its slumber, neither of us may survive.