Munou Saijo wa Akujo ni Naritai: Gimai no Migawari de Totsuida Reijou, Koushaku-sama no Dekiai ni Kizukanai

Averill, a young woman of exceptional ability who was mistreated as "incompetent" by her family. In place of her sister-in-law, Averil marries into the family of an "old, lustful duke," but the duke, Dylan, is actually a beautiful young man who is the complete opposite of what he is rumored to be. He wants to marry a "bad girl," and the contract to leave her after three years is the perfect condition for Averill's future freedom. The people around her are bewildered by the mysterious "brilliant woman" who plays the role of a "bad girl" with gusto, but they are gradually drawn to her...

https://www.cmoa.jp/title/278078/