Helfyre by Mariel Pomeroy

Two immortal creatures. One dangerous bargain.

After the death of her mother, Aheia finds herself stained by more blood than she can hide in the perpetual daylight of her home of Keloseros. She escapes the realm but pays the price: giving up her immortality and her powers–once a Mithra with light at her fingertips, she becomes near-mortal. Her desperate search for asylum leads her to the doorstep of the only place out of her abusive family’s reach—a realm that deals in death and darkness unapologetically—Aljira

But nothing in this world is free, and to buy her way to dubious safety, she’s forced to give up the only thing she has left to barter.

Her soul in exchange for seven days in the devil's realm.

Arioch, the ruler of the Nephilim, is eager to dangle his new little pet in front of the Mithra who have caused irreparable damage in his life, but he doesn’t expect the effect Aheia’s soul has on both of them once it's stamped on his arm. The pull between them is all-consuming-- they crave each other, between sharp words and angry exchanges in an increasingly passionate bond that threatens to shatter the delicate balance of everything.

Arioch and Aheia...

Keloseros and Aljira…

Even life and death.