Penda
Oswiu's relations with Penda were not entirely peaceful between 642 and 655. Bede appears to place a major assault on Bernicia by Penda, which reached the gates of Bamburgh, at some time before 651 and the death of Bishop Aidan of Lindisfarne. An entry in the Irish annals recording "he battle of Oswy against Penda" circa 650 may refer to this campaign.
D.P. Kirby suggests that the killing of Oswine may have led to an improvement in relations between Penda and Oswiu in the early 650s. Oswiu's son Ealhfrith married Penda's daughter Cyneburh, while his daughter Ealhflæd married Penda's son Peada. Peada was baptised at Ad Murum—in the region of Hadrian's Wall—by Aidan's successor Finan. Peada and Ealhflæd took a missionary group, including Cedd and Diuma, to establish a church in their lands.
In 655 Bede reports that Penda invaded Bernicia at the head of a large army. Bede states that Oswiu offered "an incalculable quantity of regalia and presents as the price of peace", but that Penda refused. Oswiu vowed to give his daughter Ælfflæd to the church, and to found a dozen monasteries if he was granted the victory, and assisted by Ealhfrith he engaged Penda with a small army in the Battle of the Winwæd, which took place in the region of Loidis, which is to say Leeds. He was successful, and Penda was killed, along with many of his allies, including King Æthelhere of the East Angles. Œthelwald had assisted Penda, but stood aside from the fighting.
The Historia Brittonum gives a somewhat different account. Here, Oswiu's offer of treasure is accepted, and is associated with the siege of a place named Iudeu. It is assumed that Ecgfrith was given over as a hostage, into the keeping of Penda's queen Cynewise, at this time. The Historia suggests that many of Penda's allies were British kings, and notes that Cadafael ap Cynfeddw joined Œthelwald in avoiding the battle, so gaining the epithet Cadomedd (the Battle-Shirker). The decisive battle is located at "Gaius's field".
Read more about this topic: Oswiu Of Northumbria