As Above An M M Demon Angel - Julie Mannino

He lowered his lips to Riley's ear. "All you have to do is be a good little pet."
When Riley's secret friend, Eren, vanishes from the woods where he hides, Riley starts to think he imagined him. After all, what little boy would have fangs, a red, pointed tail like a devil, and no need for food?
Over thirty years later, Riley is taken prisoner in the Fallen realm where angels and demons have been locked in a war for centuries. The demons keep humans and cambions as slaves to fuel their magic, there's no way out, and their horned, winged leader, Lord Erebus, looks like Eren.
At the age of forty, Lord Erebus is foul, lethal, and has a taste for dark fun. He denies their past friendship and has no time for the imaginings of a slave. All he needs is his army, his scythe, and the energy from his pet, but some glimmer of the old him is still buried under the cruelty. Even if he can't remember their past, Riley does, and maybe they can have something more even though no one else has ever earned the affection of the demon lord.
But as he uncovers more secrets about himself and Eren, Riley will learn that it never pays to love and submit to a demon.