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Bleddyn ap Cynfyn (;  AD 1075), sometimes spelled Blethyn, was an 11th-century Welsh king. Harold Godwinson and Tostig Godwinson installed him and his brother, Rhiwallon, as the co-rulers of Gwynedd on his father's death in 1063, during their destruction of the kingdom of Bleddyn's half-brother, Gruffydd ap Llywelyn. He became king of Powys and co-ruler of the Kingdom of Powys with his brother Rhiwallon from 1063 to 1075. His descendants continued to rule Powys as the House of Mathrafal.