Majo no Geboku to Maou no Tsuno

The witch known as the Eternal Vivian is dying of a mysterious illness that no known remedy can cure. With her life hanging in the balance, the daunting task of brewing the cure falls to her young apprentice, Betty, and her faithful servant, Arsenio. Yet the recipe carries a seemingly impossible requirement: one of its essential ingredients is the Horn of the Demon King—a treasure so far beyond the reach of the two novices that obtaining it borders on pure fantasy. Desperate to save their master, the unlikely pair embarks on their very first expedition to the Demon King's Castle, braving a stronghold teeming with dangers far beyond anything they have ever faced. By a stroke of luck, their arrival at the castle brings them face to face with a mysterious girl(?)—an enigmatic encounter that may prove to be the very key they need to complete the cure and save Vivian's life.