Here's a story idea.
An inexperienced angel has recently been sent from heaven for the first time to watch over a young man whose father was a powerful mage. The boy is destined to be a great mage himself. Unknown to her and to heaven, the boy has been possessed secretly by a great demon. When the angel tries to comfort his nightmares one night, she is instead drawn into them... her power stripped over the course of various tortures and given to another demon, who poses as her and deceives heaven while she is trapped in the eternal nightmare, for the great demon to play with as he wishes... she must attempt to escape the nightmare and warn heaven, but with most her power stripped from her, how will she without the demon's permission? If she does can she even be sure she truly managed to escape? Will her mind and spirit break first instead even... or will she succumb to the demon's will and become his corrupted pleasure slave, perhaps one day to be a succubus?