Oswald's father Æthelfrith was a successful Bernician ruler who, after some years in power in Bernicia, also became king of Deira, and thus was the first to rule both of the kingdoms which would come to be considered the constituent kingdoms of Northumbria ( Bernicia in the northern part and Deira in the southern part ); it would, however, be anachronistic to refer to a " Northumbrian " people or identity at this early stage, when the Bernicians and the Deirans were still clearly distinct peoples.

Oswald's mother, Acha, was a member of the Deiran royal line who Æthelfrith apparently married as part of his acquisition of Deira or consolidation of power there.

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.

" 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.

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.

After his death, Deira was subject to king Æthelfrith of Bernicia, who united the two kingdoms into Northumbria.

Æthelfrith ruled until the accession of Ælla's son Edwin, in 616 or 617, who also ruled both kingdoms until 633.

According to Bede's account in his Historia ecclesiastica gentis Anglorum ( Book I, chapter 34 ), Æthelfrith had won many victories against the Britons and was expanding his power and territory, and this concerned Áedán, who led " an immense and mighty army " against Æthelfrith.

Áedán's army also included the Cenél nEógain prince Máel Umai mac Báetáin, who is said by Irish sources to have slain Eanfrith, brother of Æthelfrith.

He was the son of Æthelfrith, a Bernician king who had also ruled Deira to the south before being killed in battle around 616 against Raedwald of East Anglia, who had given refuge to Edwin, an exiled prince of Deira.

He was the son of Eanfrith of Bernicia, who had fled into exile among the Picts after his father, Æthelfrith of Northumbria, was killed around the year 616.

One theory espoused by Cyril Hart and considered by Pauline Stafford makes her a noblewoman of Mercian stock, who descended from Ealdorman Æthelfrith of Mercia and his wife Æthelgyth, who may have been a daughter of ealdorman Æthelwulf and a niece of King Alfred's Mercian consort Ealhswith.

However, when Æthelfrith had invaded Deira, he deposed prince Edwin, heir to the throne and brother of Acha, who fled into exile.

Oswald was the son of Æthelfrith of Bernicia and came to rule after spending a period in exile ; after defeating the British ruler Cadwallon ap Cadfan, Oswald brought the two Northumbrian kingdoms of Bernicia and Deira once again under a single ruler, and promoted the spread of Christianity in Northumbria.

Northumbria was formed by Æthelfrith in central Great Britain in Anglo-Saxon times.

The kingdom reached its height under Áedán mac Gabráin ( r. 574-608 ), but its expansion was checked at the Battle of Degsastan in 603 by Æthelfrith of Northumbria.

In 547 the castle was taken by the invading Angles led by Ida son of Eoppa and was renamed Bebbanburgh by one of his successors, Æthelfrith, after Æthelfrith's wife Bebba, according to the Historia Brittonum.

Riderch was a contemporary of Áedán mac Gabráin of Dál Riata and Urien of Rheged, to whom he is linked by various traditions and tales, and also of Æthelfrith of Bernicia.

Áedán's dominance came to an end around 604, when his army, including Irish kings and Bernician exiles, was defeated by Æthelfrith at the battle of Degsastan.

His father, Æthelfrith of Bernicia, was killed in battle, fighting against Rædwald, King of the East Angles and Edwin of Deira at the River Idle in 616.

His sister Acha was married to Æthelfrith, king of neighbouring Bernicia.

During the reign of Æthelfrith, Edwin was an exile.

Bede reports that Æthelfrith tried to have Raedwald murder his unwanted rival, and that Raedwald was minded to do so, only being persuaded otherwise by his wife with Divine prompting.

Æthelfrith in battle by the river Idle in 616, and Æthelfrith was killed, along with Raedwald's son Raegenhere.

He was succeeded by Osric, son of Edwin's paternal uncle Ælfric, in Deira, and by Eanfrith, son of Æthelfrith and Edwin's sister Acha, in Bernicia.

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.

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.

The report in the Annals of Ulster for 638, " the battle of Glenn Muiresan and the besieging of Eten " ( Din Eidyn, later Edinburgh ), has been taken to represent the capture of Din Eidyn by the Northumbrian king Oswald, son of Æthelfrith, but the Annals mention neither capture, nor Northumbrians, so that this is rather a tenuous identification.

Perhaps the most significant legacies of Edwin's reign lay in his failures, the rise of Penda and of Mercia, and the return from Irish exile of the sons of Æthelfrith which tied the kingdom of Northumbria into the Irish sea world for generations.

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.

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 ".

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.

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.

The Deiran exile Hereric was poisoned while at the court of Ceretic, king of Elmet ; Æthelfrith may have been responsible for this killing.

Northumbria did not exist until the union of Bernicia with the kingdom of Deira ; this happened for the first time under Ida's grandson Æthelfrith.

The battle was a decisive victory for Æthelfrith, and Bede says, carefully, that " rom that day until the present, no king of the Irish in Britain has dared to do battle with the English.

The historian D. P. Kirby suggested that the defeat of Edwin was the outcome of a wide-ranging alliance of interests opposed to him, including the deposed Bernician line of Æthelfrith ; but considering the subsequent hostility between Cadwallon and Æthelfrith's sons, such an alliance must not have survived the battle for long.

Æthelfrith ruled both Deira and Bernicia, the two components of Northumbria, until he was killed in battle and the Deiran line was restored for a time under Edwin.

Historians have noted the marriage as evidence for Cearl's independence from the then-Northumbrian king Æthelfrith, since Edwin was Æthelfrith's rival and Cearl would not have married his daughter to an enemy of his overlord.

Edwin on the throne meant Northumbria, was no longer safe for the children of Æthelfrith as they had a potential rival claim to the throne.