King of Cumbria in Myrddin legend, probably based on a historical sixth-century ruler of Strathclyde. Nennius names him as one of the kings who fought alongside King Urien against the Saxons in the sixth century. According to the Myrddin poems, Rhydderch was one of the leaders at the battle of Arfderydd, where he fought alongside King Peredur of North Wales, and opposed King Gwenddolau of Scotland. Rhydderch was victorious. Merlin, who had fought on Gwenddolau’s side in Welsh legend, and on Rhydderch’s side in Geoffrey’s Vita Merlini, went insane at the battle and fled to the forest of Caledon. Rhydderch’s wife, Ganieda, was Merlin’s sister. In his moments of insanity, Merlin told Rhydderch that Ganieda was adulterous, but Ganieda managed to convince her husband that Merlin could not be trusted because of his madness. Rhydderch died during Merlin’s life, and his widow went to live with her brother in the forest. A Welsh poem places his grave at Abererch. A Welsh text called the “Thirteen Treasures of the Isle of Britain” names one of the treasures as a dysgl, or dish, owned by Rhydderch. It was said to provide food and drink to Rhydderch’s company and has been seen by some scholars as the origin of the Grail.