Also called the battle of Denisesburn or the battle of Cantscaul
The Battle of Heavenfield was fought between Oswald, son of Aethelfrith of Bernicia and former king of Northumbria, and Cadwallon ap Cadfan king of Gwynedd and defacto ruler of Northumbria at Heavenfield near Hexham close to Hadrian's Wall, in 633 if you follow Bede
but more likely in the year 635 after the Annales Cambriae.
The Historical Background
Oswald and his brother Oswiu 1 had been in exile in Dal Riada since the death of their father in 617 at the hands of Edwin of Deira who had supplanted Aethelfrith as ruler of Northumbria. Edwin himself died in 633 at the battle of Hatfield Moor fighting against the alliance of Cadwallon and Penda of Mercia. Cadwallon followed up the victory by occupying York ravaging the provinces of Northumbria "like a furious tyrant" 2
On Edwin's death, Eanfrith 1 came south from exile in Pictavia and seized power in Bernicia (northern Northumbria) only to meet his death at the hands of Cadwallon; likewise Edwin's cousin Osric. The future of the kingdom, so to speak, was now rested on the shoulders of young Oswald.
Not much is known about the actual battle, but it is probable that Oswald made his way south from Dal Riada and headed for Bamburgh, the established seat of power for the Bernician royal family where he could gather together what additional support was available in the north. Meanwhile Cadwallon would have set out from York and followed Dere Street northwards to Hexham.
On the eve of the battle Oswald erected a wooden cross 3 and gathered his men to pray for victory with these words;
Let us all kneel, and jointly beseech the true and living God Almighty, in his mercy, to defend us from the haughty and fierce enemy; for He knows that we have undertaken a just war for the safety of our nation. 2
He the chose a defensive position on a long sloping hill and waited for the morning.
The archaeology suggests that the battle was spread over a number of sites and at one time surged over Hadrian's Wall 4 itself; some of Cadwallon's men made a last stand at Haltington some five miles north of the Wall and Cadwallon himself died at Rowley Burn, three miles south of Hexham. Which suggests that the conflict was less of a battle and more of a series of skirmishes, which is very probable as both sides most likely only amounted to a few hundred warriors.
But the result is clear, Oswald won and Cadwallon ap Cadfan lost both the battle and his life.
The battle re-established the Bernician line of Aethelfrith as the rulers of Northumbria, but also established a line of rulers imbued with the Christian and Goedelic culture of Dal Riada. Both Oswald and his brother Oswiu (who has to succeed him as king in 642) had been raised in Iona and were baptised Christians, and they brought the Celtic tradition south. Northumbria therefore became Christian under the influence of Iona rather than Canterbury.
Since Oswald most likely owed his victory at least in part to the Dal Riadan forces that fought at his side, the battle also marked the beginning of a close political relationship between Northumbria and Dal Raida.
The defeat of Cadwallon brought to an end any hopes that the native British might stem the tide of Anglo-Saxon invasion; it was this defeat more than any other that severed the connection between the Brythonic kingdoms of North Britain and those of Wales.
1 Oswald and Oswiu where the sons of Aethelfrith by his second marriage to Acha former wife of Aella kng of Deira, Eanfrith was the son of Aethelfrith by his first marriage.
2 The Venerable Bede in the Historia Ecclesiastica Gentis Anglorum.
3 On the site of which the church of St Oswald now stands although the present church of St Oswald was rebuilt in 1717 and no trace of the original now remains.
4 Actually the only known example of any fighting at Hadrians' Wall.
The aforementioned Historia Ecclesiastica Gentis Anglorum; some details taken from
http://www.roman-britain.org/hw/heavenfield.htm which consists of a transcript of the notice-board erected at the site of the battle beside the B6318.