The Cold King's Beloved Wife is a Stand-in Princess!

Amelia, whose mother runs a high-class brothel, finds out that she is the daughter of the former king and is to marry Sigrid, the king of a neighboring country, in her cousin's place. She learns that Sigrid, who is known as a king whose hands are soaked in blood, is her first love. But she's unable to reveal the truth to him on their wedding night. "If she knew what she was getting herself into... then she's mine!" Amelia's body melts sweetly under the king's relentless, passionate lovemaking. She can see that he's still the same man she loved back then. But for some reason, Sigrid starts treating her coldly the next morning! Source: MangaPlaza