Hakushaku to yousei, book 5: Noroi no daiya ni ai wo kometeMizue Tani2005

The Fairy Doctor Lydia, is in a false engagement with her employer, the Earl of Ibrazel Edgar, due to unavoidable circumstances. With the sweet words he is so adept at, Lydia is constantly swayed around by the seductive Edgar. She is even treated as the Earl's bride by a fairy claiming to be caretaker of the moonstone on the engagement ring. On the other hand, in Edgar's vicinity - after he had gotten his hands on a suspicious black diamond - peculiar things start to happen, and it seems he plans to use this diamond as a tool for revenge against his nemesis Prince!?