In 1999, Thornton introduced the character of Dr. Dooom on the album "First Come, First Served" (1999). The album featured a track in which the character murdered Dr. Octagon. In 2002, Thornton announced "The Resurrection of Dr. Octagon", a proposed sequel to "Dr. Octagonecologyst" that would reintroduce the character. The resulting album, "The Return of Dr. Octagon" released in 2006, was largely produced without Thornton's involvement, based upon three completed vocal tracks and reconstructed outtakes. Thornton later produced "Dr. Dooom 2" in response to "The Return of Dr. Octagon".