Angel's Vengeance. Elemental Angels, #7

She's the only one of her kind. A cosmic mistake. But her greatest enemy just might be her greatest salvation. Failure and disappointment are two things Neela knows all too well. Scorned as the only female descendent of the demon ruler, Cyro-and his most egregious failed experiment to date-she longs for connection and redemption. When she finally escapes her hell, she runs into the very angel, and former prisoner of her sire, she used to care for in secret and who she hasn't forgotten. Revenge has been Rhode's bedfellow for as long as he can remember.
The fallen angel, former seraphim commander, and the latest recovering prisoner of the demon war is plagued with nightmares of what was done to him. He longs for one thing and one thing only against all demonkind: vengeance. But when an encounter with a strange and beautiful thief results in him being mortally wounded, she miraculously heals him before he realizes the cruel truth of her nature or the irrevocable bond he's powerless to stop. Rhode and Neela are flung headlong into a battle of desire and retribution where passion has the power to make lovers of enemies.
Together they must learn whether the divide between them can ever be crossed, because a new evil is coming-one that can only be defeated if their souls are strong enough to stand together.