I can’t find an old non-human story.
The general idea of the story:
A human gets saved by a demon of lust. Ends up being one of the demon kings (of lust)
She ends up being a Fell, which burns out all sin. That is the cause of all her “friends” betraying her in the past.
Said demon king of lust tries converting her to a demon without letting the other demon kings realize what she is.
Heavy bdsm undertones. Like one scene they “play” hide and seek, and she bends the rules by hanging out a window. For “breaking” the rules, she is taken right there on the bed.
Source: reddit.com/r/Erotica/comments/kwvrrn/finding_an_old_story