Helluva Servant (A Helluva Boss Side story)

This right here as the title says is a side story to the show you all know and love. Our main character is Donner, a young but very powerful demon who for years has served his master the Prince of the Ars Goetia faithfully. Now his said master has given him a very important role. To become the protector of his heir but in due time it'll spark into something much more.