[Demon Slayer] Muzan's Vicious Fiancée

Fujiwara Masako, a modern woman from the 21st century, transmigrates to feudal Heian-kyo.

Then, through a careless miscalculation, she is schemed against by the Fujiwara family's matriarch and becomes engaged to a sickly young man from the Ubuyashiki Family. In a fit of anger, she unleashes the demon Hannya and wipes out the entire family. She then moves into the Ubuyashiki Family as an orphan, intent on taking revenge on the family that dared to scheme against her alongside Lady Fujiwara.

That's right. She's a good woman who never lets a grudge go unanswered and doesn't put on airs.

Premise: Muzan's fiancee really is a good woman.