The Holy Grail of Eris

It is common knowledge that nobles will do anything for power; lying, manipulation, and extortion are the usual tactics. However, one family is exempt from this generalization: House Grail, whose main tenets are truth and integrity. The youngest daughter, Constance Grail, is just as moral as the rest of her family, believing in always doing the right thing and helping those in need.

However, these ideals do not help her when her fiancé cheats on her with another woman. Framed by his lover in the hope of her downfall, Constance has no one to hear her pleas for justice. Everyone turns a blind eye, save for one—the spirit of the infamous Scarlett Castiel—who possesses Constance and turns the tables on her accuser. However, this aid does not come without a price.

Ten years ago, Scarlett was executed for the attempted murder of Princess Cecilia. But this was actually a setup; as blunt and harsh as Scarlett could be, she would never stoop so low as to use poison. She has no idea who orchestrated her incrimination, but Scarlett demands that Constance—as the only one who can see her—help her exact revenge on all those who wronged her!

(Source: MAL Rewrite)