Blood Bonded: The Portal and Key Chronicles by Jonathan Salisbury
Eva Higgins is no stranger to violence. An assassination attempt is just a hazard of her job, especially after being blacklisted by Mr. Roper. Eva only takes mercenary jobs now that align with her own interests and moral code. With the help of her trusty assistant, Derek, she easily stays in the shadows.
But her covert skill craft is no match for Calder. He has traveled across the realms to find her. Backed by the powerful Ceann Olc and fueled by vengeance, he will stop at nothing to see her dead. With a powerful army at his command, Calder’s resources seem unlimited.