Maserfield
On 5 August 642, Penda defeated the Northumbrians at the Battle of Maserfield, which was fought near the lands of the Welsh, and Oswald was killed. Surviving Welsh poetry suggests that Penda fought in alliance with the men of Powys—apparently he was consistently allied with some of the Welsh—perhaps including Cynddylan ap Cyndrwyn, of whom it was said that "when the son of Pyb desired, how ready he was", presumably meaning that he was an ally of Penda, the son of Pybba. If the traditional identification of the battle's location with Oswestry is correct, then this would indicate that it was Oswald who had taken the offensive against Penda. It has been suggested that he was acting against "a threat posed to his domination of Mercia by a hostile alliance of Penda and Powys." According to Reginald of Durham's 12th century Life of Saint Oswald, Penda fled into Wales before the battle, at which point Oswald felt secure and sent his army away. This explanation of events has been regarded as "plausible" but is not found in any other source, and may, therefore, have been Reginald's invention.
According to Bede, Penda had Oswald's body dismembered, with his head, hands and arms being placed onto stakes (this may have had a pagan religious significance.); Oswald thereafter came to be revered as a saint, with his death in battle as a Christian king against pagans leading him to be regarded as a martyr.
Eowa was killed at Maserfield along with Oswald, although on which side he fought is unknown. It may well be that he fought as a dependent ally of Oswald against Penda. If Eowa was in fact dominant among the Mercians during the period leading up to the battle, then his death could have marked what the author of the Historia Brittonum regarded as the beginning of Penda's ten-year reign. Thus it may be that Penda prevailed not only over the Northumbrians but also over his rivals among the Mercians.
The Historia Brittonum may also be referring to this battle when it says that Penda first freed (separavit) the Mercians from the Northumbrians. This may be an important clue to the relationship between the Mercians and the Northumbrians before and during Penda's time. There may have existed a "Humbrian confederacy" that included the Mercians until Penda broke free of it. On the other hand, it has been considered unlikely that this was truly the first instance of their separation: it is significant that Cearl had married his daughter to Edwin during Edwin's exile, when Edwin was an enemy of the Northumbrian king Æthelfrith. It would seem that if Cearl was able to do this, he was not subject to Æthelfrith; thus it may be that any subject relationship only developed after the time of this marriage.
The battle left Penda with a degree of power unprecedented for a Mercian king—Kirby called him "without question the most powerful Mercian ruler so far to have emerged in the midlands" after Maserfield—and the prestige and status associated with defeating the powerful Oswald must have been very significant. Northumbria was greatly weakened as a consequence of the battle; the kingdom became fractured to some degree between Deira in its southern part and Bernicia in the north, with the Deirans acquiring a king of their own, Oswine, while in Bernicia, Oswald was succeeded by his brother, Oswiu. Mercia thus enjoyed a greatly enhanced position of strength relative to the surrounding kingdoms. Stenton wrote that the battle left Penda as "the most formidable king in England", and observed that although "there is no evidence that he ever became, or even tried to become, the lord of all the other kings of southern England ... none of them can have been his equal in reputation".
Read more about this topic: Penda Of Mercia