Geoffrey of Monmouth
The name of the historical Cadwaladr ap Cadwallon figures prominently in Geoffrey of Monmouth's romantic account of the Historia Regum Britanniae (English: History of the Kings of Britain). As such, the Cadwaladr of Geoffrey is a literary invention that used the name of a historical person in order to advance the plot of the story. In Book XII, Chapter XIV of the Historia, Cadwaladr is given as the last in a line of kings that began with Brutus of Troy. Chapters XV – XVIII have him leaving a depopulated Britain for Brittany, then traveling to Rome, where he dies after meeting the pope.
Also traced to Geoffrey's fertile imagination are stories of Ivor ap Alan and Ynyr traveling from Brittany to Britain. The choice of names for Ivor and Ynyr in the stories may be a consequence of spurious additions to the Laws of Edward the Confessor, which inaccurately speak of good relations between Wessex and the Welsh in the reign of King Ine of Wessex (reigned 688 – 726). From there emerges a conflation of Cadwaladr or Cadwallon with Cædwalla of Wessex (reigned 685 – 688), and a conflation of Cadwaladr's son Ivor with Cædwalla's son Ine.
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