king of Bernicia (592-604)
king of Northumbria (604-617)
Aethelfrith was the son of Aethelric and grandson of Ida the reputed founder of the kingdom of Bernicia, and was established as king of Bernicia by 592, although we know nothing of the circumstances of his accession to the throne.
If the battle of Catraeth, which is commemorated in the Welsh poem Gododdin, actually occurred and that the identification of the location battle at Catterick is correct then it would presumably have been against Aethelfrith that the battle was fought. The dating of this possible battle is uncertain, but a date around 600 is most likely.
What is reasonably certain is that in 603 Aidan, the king of Dalriada (in what is now western Scotland), raised an army and marched south against Bernicia. At the battle of Daegsanstane (2), Aethelfrith's brother Theobald was killed, but most of the Dalriadan army was wiped out. (3)
Encouraged by this victory, in 604 Aethelfrith made the key decision of his reign to expand his rule by taking over the neighbouring kingdom of Deira. He achieved this by the simple expedient of killing the incumbent king, who was most probably Aethelric son of Aelle(and a different Aethelric from the one that was his own father). To consolidate his position he then married Aethelric's daughter, the princess Acha, but her brother Edwin the Deira heir, made his escape.
Almost ten years later he led his army across the country to the old Roman fortress of Chester were he fought and won a battle against the Welsh forces of Selyf ap Cynan, king of Powys. No one is quite sure what Aethelfrith was doing that far west; possibly he was seeking to prevent any Welsh interference with plans for South Rheged or perhaps it was a move against Mercia where Edwin had taken refuge, designed to frustrate any alliance between Mercia and the Welsh kings.
In any event Edwin seems to have then fled Mercia for the comparative safety of the kingdom of East Anglia. Aethelfrith, no doubt keen to remove Edwin from the picture, sought to bribe Raedwald (who was the East Anglian king) to have him killed, but Raedwald declined and responded by raising an army. Together Raedwald and Edwin marched against Aethelfrith and catching him by surprise at the battle of the River Idle in around 617, they succeeded in both defeating and killing him. Aethelfrith's sons fled north into exile, and Edwin assumed power in Northumbria.
Described by Bede as "a very brave king and most eager for glory" , which about sums all we know of Aethelfrith's life, one long succession of battles fought. It is probable that at the time Aethelfrith assumed power that Bernicia was little more than a little coastal strip of territory based around the fortress of Bamburgh (4); by the time of his death twenty five years later, his aggressive expansionism had established Northumbria as one of the most powerful kingdoms in Britain.
(1) Also known as Æthelfrith or Ethelfrith or Ethelfrid or even variants thereof due to the variability inherent in transcribing Anglo-Saxon into modern English.
(2) Although the site of this battle is sometimes transcribed as Theakstone, and locations such as Dawston and Liddlesdale have been suggested, I can only conclude that no one knows for sure where this was fought.
(3) The Anglo-Saxon Chronicle adds that "Hering, the son of Hussa, led the army thither".
Since there was a Hussa, son of Ida, that was himself king of Bernicia, and Aethelfrith's older brother, it might well be the Hering regarded himself as the rightful heir, had fled to exile in Dalriada and had sought Aidan's assistance to gain the throne of Bernicia.
After all this was exactly what happened with Oswald, (Aethelfrith's son) a generation later.
This might be a logical answer to the question, what the heck was Dalriada doing invading Bernicia? The standard explanation appears to be that Dalriada was responding to some perceived BerniciaBernician] threat. One that I have never found convincing as I have never believed that Bernicia was in a position to threaten Dalriada. But this is merely my speculation.
(4) In fact, according to the Historia Brittonum it was Aethelfrith who was responsible for the renaming the coastal stronghold of Dinguoaroy as Bebbanburh in honour of his wife Bebbe. (A name that later morphed into the modern Bamburgh)