Oswald was the son of Æthelfrith of Bernicia and Acha of Deira and came to rule after spending a period in exile. After defeating the Welsh Gwyneddian ruler, Cadwallon ap Cadfan, at the Battle of Heavenfield, 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.