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

Despite her hidden genius, Avril has lived a life of misery, constantly belittled as useless by her own family.

When she is forced to substitute for her wicked step-sister in a marriage to a supposedly lecherous, old duke, Avril expects the worst. But surprisingly, Duke Dylan is actually a breathtakingly handsome young man.

He offers her a deal: act as his villainess wife for three years, and then they will divorce. For Avril, this contract is a golden ticket to her ultimate freedom. But as she enthusiastically embraces her new evil persona, her mysterious charm leaves the duke and his household completely bewildered and entirely enchanted.