He is voiced by himself. The Undertaker was first seen when Nicky Jr. was possessed by Captain Doody (a parody of Pazuzu who has also a nickname, Captain Howdy), when Doody used Nick Jr. as his vessel to battle The Undertaker, Taker manages to exorcise this demon out of Nicky Junior's body, Nicky Jr. who has no memory for being possessed by Captain Doody reunited with his father Nick Sr. which make The Undertaker impressive.