Akuyaku Reijou To Kichiku Kishi summary:

From Painful Nightz: The Marquis' daughter, Cecilia, had suddenly realised the fact that she had reincarnated into an Otome game as the villainous daughter! However, it was already too late for her to avoid condemnation. After breaking off her marriage engagement, she was framed by the heroine and had fallen, forcing her to become a prostitute. Her first customer was the heroine’s childhood friend, the Deputy Leader of the Imperial Guards, Lucas Herbst, who was so strong that he could even be considered a cheater in life. Frightened, Cecilia was embraced relentlessly only to awake in the residence of the Duke of Herbst the next morning. Moreover, a mark of the vow was engraved on her lower abdomen, marking her as Lucas’s fiancéeー?!