Mordred is a Goa'uld and one of Manannan mac Lirs underlord. When MacAlester St. John was sent on a mission to planet a Longinus. disguised himself as a cupbearer known as Peter Doyle which had been donated to the Observation Dome of Mordred. (RPG: "First Steps: The Stargate Unexplored Worlds Roleplaying Sourcebook")

By some accounts, Mordred was actually a Goa'uld, son of System Lord Manannan mac Lir. Manannan holding on Earth included the area where Arthur resided, and Arthur raised a rebellion against the Goa'uld rule. Manannan's queen, Morgause seduced Arthur and her human values ​​became pregnant with a child. The child was implanted with a Goa'uld, Mordred, who would go on to kill Arthur and end their uprising. He would go on to become his father's head during the Lord, to be both an expert warriors and skilled tactician. Manannan keeps a watchful eye on his talented son, waiting for the inevitable betrayal.(RPG: "Living Gods: Stargate System Lords")