Æthelfrith of Northumbria Æthelfrith ( died c . 616 ) was King of Bernicia from c . 593 until c . 616 ; he was also , beginning c . 604 , the first Bernician king to also rule Deira , to the south of Bernicia . Since Deira and Bernicia were the two basic components of what would later be defined as Northumbria , Æthelfrith can be considered , in historical terms , the first Northumbrian king . He was especially notable for his successes against the Britons and his victory over the Gaels of Dál Riata . Although he was defeated and killed in battle and replaced by a dynastic rival , his line was eventually restored to power in the 630s . Background . Æthelfrith , son of Æthelric and grandson of Ida , apparently succeeded Hussa as king of the Bernicians around the year 592 or 593 ; Æthelfrith ' s accession may have involved dynastic rivalry and the exile of Hussa ' s relatives . The genealogies attached to some manuscripts of the " Historia Brittonum " say that Æthelfrith ruled Bernicia for twelve years and ruled Deira for another twelve years , which can be taken to mean that he ruled in Bernicia alone from about 592 to 604 , at which point he also came to the throne of Deira . His predecessors are obscure ; Æthelfrith is the earliest Bernician ruler about whom any significant details are known . The 20th - century historian Frank Stenton wrote that " the continuous history of Northumbria , and indeed of England , begins with the reign of Æthelfrith " , and that " he was the real founder of the historic Northumbrian kingdom , and he was remembered as the first great leader who had arisen among the northern Angles . " Conquests . Bede tells of Æthelfrith ' s great successes over the Britons , while also noting his paganism ( the conversion of Northumbria did not begin until a decade after his death ) : he " ravaged the Britons more than all the great men of the English , insomuch that he might be compared to Saul , once king of the Israelites , excepting only this , that he was ignorant of the true religion . For he conquered more territories from the Britons , either making them tributary , or driving the inhabitants clean out , and planting English in their places , than any other king or tribune . " It may have been Æthelfrith who destroyed the British army at the Battle of Catraeth ( Catterick , c . 600 ) ; the battle is described in the early poem Gododdin . The Britons called him " Flesaur " , or " the twister " . It was under Æthelfrith that Bernicia ' s boundaries pushed significantly inland from the coast , and penetrated further into British territory . Áedán mac Gabráin , the Irish king of Dál Riata ( to the northwest of Bernicia ) , was alarmed by Æthelfrith ' s successes , and in 603 he led " an immense and mighty army " against Æthelfrith . Although Æthelfrith commanded an inferior force , according to Bede , he won a crushing victory at a place called Degsastan ; most of Áedán ' s army was killed , and Áedán himself fled . Bede says that Æthelfrith ' s victory was so great that the Irish kings in Britain would not make war on the English again , right up to Bede ' s own time . The battle appears to have been costly for Æthelfrith as well , however ; Bede says that Æthelfrith ' s brother Theodbald was killed , " with almost all the forces he commanded " . The appearance of Hering , son of Hussa , Æthelfrith ' s predecessor , on the side of the invaders seems to indicate dynastic rivalry among the Bernicians . Æthelfrith may have come to terms with the Irish of Dál Riata after this , judging from the fact that Æthelfrith ' s subsequent known military campaigns took place in other parts of Britain ; that his sons were later able to take refuge among the Irish of Dál Riata after Æthelfrith ' s own death in battle may be significant . Æthelfrith gained control of Deira around 604 ; the circumstances of this are unknown . That he gained Deira through conquest is suggested by the exile of Edwin , son of the former king Ælla , and Hereric , Edwin ' s nephew , who were both notable members of the Deiran royal line ; the short five - year reign of Aethelric of Deira , who ruled immediately prior to Æthelfrith ' s acquisition of Deira , may also indicate conquest . On the other hand , D . P . Kirby suggested that Æthelfrith ' s rule of both kingdoms may have represented " a formalization of an existing relationship " of cooperation between the two . Kirby also pointed out that Edwin did not necessarily go into exile immediately , and considered it likely that Æthelfrith ' s hostility towards him " manifested itself only by degrees " . Edwin , apparently seeking safety from Æthelfrith , seems to have travelled between many different kingdoms during his period of exile . He may have spent time during his exile in the British kingdom of Gwynedd , and it seems clear that he spent time in Mercia , because he married a daughter of king Cearl . Ultimately , he took refuge in East Anglia , where his presence precipitated the events that caused Æthelfrith ' s downfall . It was also around 604 that Æthelfrith ' s son Oswald was born . Oswald ' s mother was Acha , daughter of Ælla , and thus Edwin ' s sister . Although Bede does not explicitly say Æthelfrith married Acha , it is thought that he did so ; he may have married her prior to taking power in Deira , in which case the marriage may have facilitated it , or he may have done so afterwards in order to consolidate his position there . The " Historia Brittonum " says that Æthelfrith gave the town of Din Guaire to his wife Bebba , after whom it was named Bamburgh ; Bede also says that Bamburgh was named after a former queen named Bebba , although he does not mention Æthelfrith . It has been suggested that she " was probably Æthelfrith ' s first and most important wife " . Later in his reign , probably between 613 and 616 , Æthelfrith attacked the Kingdom of Powys and defeated its army in a battle at Chester , in which the Powysian king Selyf Sarffgadau was killed , along with another king called Cetula , who was probably Cadwal Crysban of Rhôs . He also massacred the monks of Bangor - Is - Coed who were assembled to aid the Britons by their prayer s . Bede says that he decided to attack them because , although they were not armed , they were opposing him through their prayers . The number of dead monks was said to be about 1200 , with only fifty escaping . It has been suggested that Æthelfrith may have done this for tactical reasons , to catch the Britons by surprise and force them to change their plans in order to protect the monks . After first killing the monks , Æthelfrith prevailed over the enemy army , although Bede notes that Æthelfrith ' s own forces suffered considerable loss . Æthelfrith ' s victory at Chester has been seen as having great strategic importance , as it may have resulted in the separation of the Britons between those in Wales and those to the north ; however , Stenton noted that Bede was mainly concerned with the massacre of the monks and does not indicate that he regarded the battle as a historical " turning - point " . Koch says that the older view that the battle cut the two British areas off from each other is now " generally understood " to be outdated , as Æthelfrith died soon after , and there is " almost no archaeological evidence for Anglo - Saxon settlement within the pagan period in Cheshire or Lancashire " , and in any case the sea would have been the primary means of communication . Rivals . The Deiran exile Hereric was poisoned while at the court of Ceretic , king of Elmet ; Æthelfrith may have been responsible for this killing . Edwin ended up in East Anglia , under the protection of its king , Raedwald . Æthelfrith sent messengers to bribe Raedwald with " a great sum of money " into killing Edwin ; Bede reports that his first message had no effect , but Æthelfrith sent more messengers and threatened war if Raedwald did not comply ( bribes and threats of this kind may have previously been used to accomplish Hereric ' s killing . ) Raedwald eventually agreed to kill Edwin or hand him over to Æthelfrith ' s messengers , but was reportedly dissuaded from this by his wife , who said that such a thing was unworthy of his honour . Instead , Raedwald raised an army and marched against Æthelfrith , and around 616 Æthelfrith was defeated and killed on the east side of the River Idle by an army under Raedwald ; Bede says that Æthelfrith had the inferior army , because Raedwald had not given him time to bring all his forces together . While presented by Bede as being fought simply over the issue of Edwin , this war may have actually involved questions of power and territory between the two rulers . Following Æthelfrith ' s death , Edwin became king not just of Deira but of Bernicia as well ; Æthelfrith ' s sons Eanfrith , Oswald , and Oswiu fled to the north . Thus Æthelfrith ' s death in battle has been seen as causing " a near total revolution in the politics of what is now northern England " . After Edwin was killed in 633 at the Battle of Hatfield Chase , Eanfrith temporarily regained power in Bernicia , and subsequently Oswald restored the Bernician line of Æthelfrith to power in both Bernicia and Deira . After this point , Æthelfrith ' s descendants continued to rule until the first part of the eighth century . Family . Æthelfrith , according to the 9th - century Anglo - Saxon genealogies ( of doubtful historicity ) was the son of Æthelric and grandson of Ida . Æthelfrith married Acha of Deira , daughter of Ælla of Deira . They had eight children :