Oswald (; c 604 – 5 August 641/642) was King of Northumbria from 634 until his death, and is venerated as a saint, of whom there was a particular cult in the Middle Ages. Oswald was the son of Æthelfrith of Bernicia and came to rule after spending a period in exile. After defeating the Welsh 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. He was given a strongly positive assessment by the historian Bede, writing a little less than a century after Oswald's death, who regarded Oswald as a saintly king; it is also Bede who is the main source for present-day historical knowledge of Oswald. After eight years of rule, in which he was the most powerful ruler in Britain, Oswald was killed in the Battle of Maserfield while fighting the forces of Penda of Mercia, who then himself was defeated by Oswald's brother Oswiu. Background, youth, and exile 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. 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 of Deira, was a member of the Deiran royal line whom Æthelfrith apparently married as part of his acquisition of Deira or consolidation of power there. Oswald was apparently born in or around the year 604, since Bede says that he was killed at the age of 38 in 642; Æthelfrith's acquisition of Deira is also believed to have occurred around 604. Æthelfrith, who was for years a successful war-leader, especially against the native British, was eventually killed in battle around 616 by Raedwald of East Anglia at the River Idle. This defeat meant that an exiled member of the Deiran royal line, Edwin (Acha's brother), became king of Northumbria, and Oswald and his brothers fled to the north. Oswald thus spent the remainder of his youth in the Scottish kingdom of Dál Riata in northern Britain, where he was converted to Christianity. He may also have fought in Ireland during this period of exile.