The The Holy Grail of Eris, Vol. 2 (light novel) — Kujira Tokiwa

Thanks to aid from the ghost of Scarlett Castiel, Constance escaped the incident at the Grand Merillian as the triumphant victim. Good deeds don't come for free however, and the price for Scarlett's help is vengeance! Although she was unabashedly wicked, the villainess is adamant that she was framed for poisoning the crown princess -- but with no evidence, Constance has her work cut out for her. Perhaps the strange circumstances behind the apparent suicide of Lady Lily Orlamunde might provide some clues…