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Cynfarch Oer ('Cynfarch the Dismal', also known as Cunomarcus or Cynfarch ap Meirchion) was probably a 6th-century king of the Sub-Roman realm of Rheged, believed to be located in north-west England and south-west Scotland.
Next to nothing is known about Cynfarch. He appears in the Old Welsh pedigrees of the Brythonic 'Men of the North' as the son of the equally obscure Meirchion Gul (Marcianus the Lean) and father of the slightly better documented Urien Rheged. His name was well remembered however and his family were known as the 'Cynferchyn' in his honour. His unflattering epithet Oer probably means '(the) Unwelcoming' (literally 'cold').
It is assumed that Cynfarch ruled in Rheged before Urien. Hence the apparent 'Rheged' placename at Dunragit near Stranraer and tales of March ap Meirchion (or Cun-march ap Meirchion) in the same area. He may also have been a participant in the Battle of Arfderydd in 573.