The Holy Grail of Eris

Constance Grail, known as Connie, is a plain and ordinary Viscount's daughter, and Scarlet Castiel, the daughter of a duke who was once executed as an unparalleled villain. Connie is saved from a difficult situation by Scarlet's ghost, but in exchange, she must continue to accompany her on her quest for revenge. 

Due to certain circumstances, Connie and her friends end up visiting Elsor, the homeland of Scarlet's late mother. According to Scarlett, this is where "the Star Crown of the Faris Empire's last princess rests." 

(Source: Drecom, translated)

As 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 fiance 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! 

(Source: Yen Press)