In The Holy Grail of Eris, noble houses are expected to lie, manipulate, and extort their way to power, but House Grail remains bound to a strict code of truth and integrity. Constance Grail, the youngest daughter of that family, has been raised to believe that doing the right thing and helping those in need still matters, even in a world where status decides almost everything.
That belief collapses when her fiancé betrays her with another woman and that lover frames Constance in a scheme meant to destroy her reputation. With no one willing to listen, Constance is abandoned by the people around her until the spirit of the infamous Scarlet Castiel intervenes and takes over her body, forcing the balance to turn against Constance's accuser. Scarlet's help, however, comes at a price. Ten years earlier, she was executed for the attempted murder of Princess Cecilia, but the accusation was part of a setup; harsh and blunt as Scarlet may have been, she insists she would never resort to poison.
Unable to identify the hand behind her downfall, Scarlet forms a pact with Constance, the only person who can see her, and demands her aid in uncovering the truth and taking revenge on everyone who wronged her. Together, the two women enter a dangerous game of masks, class politics, and old grudges, where exposing one lie only reveals another.
