The Substitute Witch Challenges the Cold-Hearted King

[Complete] The young lady who was designated as the Crown Prince's fiancée was troubled by a certain secret. To help her, the witch Shellasa agrees to become a temporary substitute bride with the Crown Prince's consent. The witch had said it sounded interesting, but after meeting King Evilson, who is called the "Cold-Blooded King" in the royal palace, things begin to change...!? (Or, a story where the heroine goes to her boyfriend's family home as a substitute fiancée for her friend, only to find a super attractive father-in-law, and while hiding her own secret, she secretly admires him.)