2x18 - The Grim Adventures of Billy and Mandy

Billy and Mandy are sent to "Winter Camp", where they meet Nergal's son, Nergal Jr. Just like his father, Junior tries making friends, although not in the best way possible. Junior attacks the campers, freezes them and, using his shape-shifting abilities, takes over their appearances. Billy decides to be Junior's friend, but he is scared away at the moment Junior shows him his true form.