Prayers of the Vulgar
Wow that's was so stupid for a Christian mythology heavily inspired and awaist of time So ML is an evil entity that got reborn as a human, and FL need to make sure that it doesn't wake up until the human death ... But he was hiding his power to be with her and she end up killing him once it was revealed... But she exchange he anglic being to get his human life back, unfortunately he doesn't remember her... But actually he made a deal with her mother to pretend to lose his memory for 10 years ... The smut was bad, the first 1/3 of the story was a waist of words and it wasn't that exciting or interesting or taboo or anything thing at most probably boring.
Osagashi no Hatsukoi Aite wa Tabun Watashi Desu, to wa Totemo Ienai.