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 their half-brother, Gruffydd ap Llywelyn. Bleddyn became king of Powys and co-ruler of the Kingdom of Gwynedd with his brother Rhiwallon from 1063 to 1075. His descendants continued to rule Powys as the House of Mathrafal. Background Bleddyn was born to a poorly documented Powys nobleman named Cynfyn ap Gwerystan, known only from the late traditional pedigrees reporting Bleddyn's parentage. Cynfyn's claimed father, Gwerstan or Gwerystan, is given contradictory Welsh pedigrees consisting mostly of otherwise unknown names, a possibly spurious derivation since his name perhaps actually represents a rendering of the Anglo-Saxon name Werestan. Cynfyn, likely a supporter of king Llywelyn ap Seisyll, would after the latter's 1023 death marry the widowed queen, Angharad, daughter of King Maredudd of Dyfed, whose realm had been lost to the Irish pretender Rhain before its conquest by Llywelyn. Angharad and Cynfyn had at least two sons, Bleddyn and Rhiwallon, probably born in the late 1020s, who were thus maternal half-brothers of Gruffydd ap Llywelyn, Angharad's son by her first husband. Gruffydd, aged about ten and passed over for succession at the time of his father's death, slowly rebuilt his father's realm, annexing its successor states. Cynfyn and Rhiwallon first appear in the documentary record in 1063. Bleddyn may have been residing in Powys, where he married Haer ferch Cillyn, daughter of the Lord of Gest Cillyn y Blaidd Rudd ("Cillyn the Red Wolf"). Reign Gruffydd's consolidation of power and alliance with Ælfgar of Mercia made him a threat to Harold Godwinson, earl of Hereford. Upon Ælfgar's death in 1060, Harold and his brother Tostig quickly invaded; the following year, they invaded again and were left in mastery of Wales after traitors among his men killed Gruffydd during a retreat. The south was restored to the Houses of Dinefwr and Morgan, but Powys and Gwynedd were given to Gruffydd's half-brothers Bleddyn and Rhiwallon. These two submitted to Harold and swore themselves vassals and allies of Edward the Confessor. Closely allied with Harold, the brothers joined the Saxon resistance to William the Conqueror following his conquest of England. In 1067, they joined the Mercian Eadric the Wild in his attack on Norman