Wikipedia
Cunedagius ( Latinized form; ) was a legendary king of the Britons, as recounted by Geoffrey of Monmouth. He was the son of Henwinus, Duke of Cornwall, and Regan, the daughter of King Leir.
Cunedagius, grandson of Leir, despised the rule of his aunt Cordelia. With the help of his cousin Marganus, Cunedagius took over the kingdom from Cordelia and ruled half of it. Following Cordelia's suicide, Cunedagius came to rule the region of Britain southwest of the Humber.
Two years after they split the island, Marganus invaded Cornwall and destroyed much of the land. Cunedagius met him in battle and defeated him. Marganus fled throughout Britain until he was cornered in Wales. Cunedagius killed him and became king of all of Britain. He ruled all of Britain for 33 years and was succeeded by his son, Rivallo.
Geoffrey synchronizes Cunedagius' reign with the ministry of the Jewish prophet Isaiah and the founding of Rome by Romulus and Remus. Both events are dated to the 8th century BC.