Daughter of Darkness by Juliana Haygert

She is destined to kill a demon.
He is sent to protect her.
One of them is about to fail …

He spent centuries in the dark …
After 300 years spent in the underworld as a punishment, warrior Devon is called on by the gods for a chance to fix his mistake—and to win his redemption. But there’s a catch: the gods won’t tell him details about his mission. He’s supposed to suffer while trying to figure out what he did wrong in the past, and fix it in the present.
All Kenna ever wanted was to be the owner of her own life and destiny. But for now all she can do is run away from the evil wanting to claim her powers.

… until she came …

When Devon becomes unintentionally entangled in his new neighbor’s life, he can’t help but feel he’s closer to his purpose. Every moment he spends with Kenna makes him confused, and every time he touches her, glimpses of the past, of his failed mission, come back to haunt him.

… and showed him the light.

Darkness is closing in, and with Kenna’s help, Devon needs to put the pieces of the puzzle together before he fails his mission again and evil consumes the world.
Only this time, he’s sure he won’t be the only one damned.

**Upper YA/NA Contemporary Fantasy Romance.