Excerpts from Books and Wikipedia
"The Battle of Heavenfield was fought in 633 or 634 between a Northumbrian army under Oswald of Bernicia and a Welsh army under Cadwallon ap Cadfan of Gwynedd. The battle resulted in a decisive Northumbrian victory. . . . An alliance between Cadwallon of Gwynedd and King Penda of Mercia had led to an invasion of Northumbria. This was an odd alliance between a Christian king of Brythonic descent and a pagan king of Anglian descent. . . . After the battle, Oswald re-united Deira with Bernicia and became king of all Northumbria. Bede believed that the importance of the battle was that it restored Christianity to Northumbria. Oswald spent eight years upon the Northumbrian throne before he was killed in the Battle of Maserfield. [Wikipedia]