Majo no Geboku to Maou no Tsuno

The Eternal Vivian is dying from an unknown disease and must rely on her young apprentice Betty and her servant Arsenio to make the cure. Unfortunately, one of the ingredients is Horn of the Demon King- something completely out of reach of the two. Luckily, on the first expedition to the Demon King's Castle they meet a girl(?) who may be able to help them out...