Absolute Surrender by Georgia Lyn Hunter — book slip

Born in the flames of Heaven's wrath, Aethan is an immortal guardian who has walked alone for millennia, trapped in a hell of his own making.

His solitary existence is shattered when he encounters Echo Carter, a feisty mortal who can see demons and is determined to hunt down the demonii responsible for her friend's death.

Despite the danger, Echo is drawn to Aethan, and their collision ignites a dangerous passion. As evil closes in, Aethan must confront his darkest fears to claim Echo as his own, or risk losing her to a sinister fate.

First in the 'Fallen Guardians' series.