Le Chevalier D'Eon

D'Eon de Beaumont, knight of France and member of the secret police, finds his sister's corpse floating in a coffin down the Seine River, with the words "Psalms" written on its lid. In the journey to search for the reason of his sister's death/murder, he uncovers the mysteries of the Psalm of the Royal Family with the aid of three other companions, and of course- his sister's vengeful soul, which possesses his body.