Darkest Heart by Juliette Cross

Wrong side of heaven…right side of redemption.

Anya—a stoic, blue-winged angelic warrior— was bitten by a demon prince in battle, and now she has precious little time to find a cure for his deadly venom. But the only archangel with the power to stop the dark poison from corrupting her body and soul is missing. She’ll have to trust her guide, the outcast high demon Dommiel, who is as handsome as he is dangerous if she has any hope.

An outcast of his own kind, high demon Dommiel stays under cover while the war between angels and demons rages on. But when the only person who ever showed him kindness asks for his help, he has no choice but to try to save the angel. Venturing back into the dens he has avoided for so long, Anya makes him want and feel things he never thought possible.

But Dommiel knows there is no way an angel can ever love a demon…