Cadwallon ap Cadfan Cadwallon ap Cadfan ( died 634 ) was the King of Gwynedd from around 625 until his death in battle . The son and successor of Cadfan ap Iago , he is best remembered as the King of the Britons who invaded and conquered Northumbria , defeating and killing its king , Edwin , prior to his own death in battle against Oswald of Bernicia . His conquest of Northumbria , which he held for a year or two after Edwin died , made him the last Briton to hold substantial territory in eastern Britain until the rise of the Tudor dynasty . He was thereafter remembered as a national hero by the Britons and as a tyrant by the Anglo - Saxons of Northumbria . History . As with other figures of the era little is certainly known of Cadwallon ' s early life or reign . The primary source of information about him is the " Historia ecclesiastica gentis Anglorum " of the Anglo - Saxon writer Bede , who is strongly critical of him . Cadwallon consistently appears in the genealogies of the Kings of Gwynedd as the son of Cadfan ap Iago and a descendant of Maelgwn Gwynedd and Cunedda . Historian Alex Woolf , however , presents the case that the genealogists have erroneously inserted Bede ' s Cadwallon into the pedigree of the unrelated Kings of Gwynedd as son of Cadfan . Instead , Woolf suggests that Bede ' s Cadwallon was the " Catguallaun liu " found in genealogies as son of " Guitcun " and grandson of Sawyl Penuchel , rulers in the " Hen Ogledd " or Brythonic - speaking area of northern Britain . Whatever the case may be , Cadwallon was certainly affected by the ambitions of Edwin , King of Northumbria . Bede , writing about a century after Cadwallon ' s death , describes Edwin , the most powerful king in Britain , conquering the Brythonic kingdom of Elmet ( what is now western Yorkshire ) and ejecting its king , Cerdic . This opened the door to the Irish Sea , and Edwin successfully extended his rule to the " Mevanian Islands " – the Isle of Man and Anglesey . The " Annales Cambriae " says that Cadwallon was besieged at Glannauc ( Priestholm , or Puffin Island ) , a small island off eastern Anglesey , and dates this to 629 . Surviving Welsh poetry and the Welsh Triads portray Cadwallon as a heroic leader against Edwin . They refer to a battle at Digoll ( Long Mountain ) and mention that Cadwallon spent time in Ireland before returning to Britain to defeat Edwin . According to Geoffrey of Monmouth ' s " History of the Kings of Britain " ( which includes a fairly extensive account of Cadwallon ' s life but is largely legendary — for example , Geoffrey has Cadwallon surviving until after the Battle of the Winwaed in 654 or 655 ) , Cadwallon went to Ireland and then to the island of Guernsey . From there , according to Geoffrey , Cadwallon led an army into Dumnonia , where he encountered and defeated the Mercians besieging Exeter , and forced their king , Penda , into an alliance . Geoffrey also reports that Cadwallon married a half - sister of Penda . However , his history is , on this as well as all matters , suspect , and it should be treated with caution . In any case , Penda and Cadwallon together made war against the Northumbrians . A battle was fought at Hatfield Chase on October 12 , 633 which ended in the defeat and death of Edwin and his son Osfrith . After this , the Kingdom of Northumbria fell into disarray , divided between its sub - kingdoms of Deira and Bernicia , but the war continued : according to the " Anglo - Saxon Chronicle " , " Cadwallon and Penda went and did for the whole land of Northumbria " . Bede says that Cadwallon was besieged by the new king of Deira , Osric , " in a strong town " ; Cadwallon , however , " sallied out on a sudden with all his forces , by surprise , and destroyed him [ Osric ] and all his army . " After this , according to Bede , Cadwallon ruled over the " provinces of the Northumbrians " for a year , " not like a victorious king , but like a rapacious and bloody tyrant . " Furthermore , Bede tells us that Cadwallon , " though he bore the name and professed himself a Christian , was so barbarous in his disposition and behaviour , that he neither spared the female sex , nor the innocent age of children , but with savage cruelty put them to tormenting deaths , ravaging all their country for a long time , and resolving to cut off all the race of the English within the borders of Britain . " Bede ' s extremely negative portrayal of Cadwallon as a genocidal tyrant cannot be taken at face value . Cadwallon ' s alliance with the Saxon Penda undermines Bede ' s assertion that Cadwallon had attempted to exterminate the Saxons . Additionally , the fact that Cædwalla , king of Wessex a generation after Cadwallon ' s death , bore a name derived directly from the British " Cadwallon " suggests that Cadwallon ' s reputation could not have been so poor among the Saxons of Wessex as it was in Northumbria . Still it is unlikely that Bede , a careful historian , made up the story entirely . He may have been influenced by Welsh prophetic poetry such as the " Armes Prydain " in which the Brytons are predicted to finally succeed in forcing the Saxons from Britain for good . The new king of Bernicia , Eanfrith , was also killed by Cadwallon when the former went to him in an attempt to negotiate peace . However , Cadwallon was defeated by an army under Eanfrith ' s brother , Oswald , at the Battle of Heavenfield , " though he had most numerous forces , which he boasted nothing could withstand " . Cadwallon was killed at a place called " Denis ' s - brook " .