The Villainess And The Demon Knight (Manga) Volume. 1

Cecelia, daughter of a grand Marquis, one day realizes she's actually a reincarnated being, born into the world of an otome game as a villainous noble. However, she's been reincarnated after the fall, leaving her framed and forced into sex work at a brothel. And just when things couldn't get worse, Cecelia's first customer is Lucas Herbst, the Deputy Leader of the Imperial Guards: a Casanova so strong and fierce that when he steals Cecelia away for a night of debauchery, she's sure this is certain doom. The next day, to her surprise, she wakes up with the crest of a vow on her stomach marking her as Lucas's... fianc ?!