The Holy Grail of Eris, Vol. 1 (light novel) - Kujira Tokiwa, Yu-Nagi Yu-Nagi

Never neglect to read the fine printAs a viscount’s daughter, Constance Grail is an ordinary girl whose only notable trait is sincerity, leaving her with no real options when someone steals her fianc  and falsely accuses her of petty theft at a ball. While Connie awaits her undeserved punishment, a ghost appears to offer a bargain. The spirit is none other than Scarlett Castiel, a noblewoman once praised for her beauty, lineage, and undeniable charisma-and executed some ten years earlier for her wicked deeds. When Connie accepts this proposal, she is possessed by the infamous spirit who instantly turns the tide on her tormenter, thus saving the hapless girl from her miserable fate. It’s never wise striking a deal without knowing exactly what you’ve committed to, though...or you might find yourself bound to a ghost dead set on revenge against her own enemies!