It has been considered that Oswald is one of the three Saxon princes mentioned in the Irish poem Togail Bruidne Dá Derga, being named as 'Osalt' in that work. Victory over Cadwallon After Cadwallon ap Cadfan, the king of Gwynedd, in alliance with the pagan Penda of Mercia, killed Edwin of Deira in battle at Hatfield Chase in 633 (or 632, depending on when the years used by Bede are considered to have begun), Northumbria was split into its constituent kingdoms of Bernicia and Deira. Oswald's brother Eanfrith became king of Bernicia, but he was killed by Cadwallon in 634 (or 633) after attempting to negotiate peace. Subsequently, Oswald, at the head of a small army (possibly with the aid of allies from the north, the Scots and/or the Picts), met Cadwallon in battle at Heavenfield, near Hexham. Before the battle, tradition says Oswald had a wooden cross erected; he knelt down, holding the cross in position until enough earth had been thrown in the hole to make it stand firm. He then prayed and asked his army to join in. Adomnán in his Life of Saint Columba offers a longer account, which Abbot Ségéne had heard from Oswald himself. Oswald, he says, had a vision of Columba the night before the battle, in which he was toldBe strong and act manfully. Behold, I will be with thee. This coming night go out from your camp into battle, for the Lord has granted me that at this time your foes shall be put to flight and Cadwallon your enemy shall be delivered into your hands and you shall return victorious after battle and reign happily. Oswald described his vision to his council and all agreed that they would be baptised and accept Christianity after the battle. In the battle that followed, the Welsh (Brytons) were routed despite their superior numbers; Cadwallon himself was killed. Overlordship Following the victory at Heavenfield, Oswald reunited Northumbria and re-established the Bernician supremacy which had been interrupted by Edwin. Bede says that Oswald held imperium for the eight years of his rule (both Bede and the Anglo-Saxon Chronicle say that Oswald's reign was actually considered to be nine years, the ninth year being accounted for by assigning to Oswald the year preceding his rule, "on account of the heathenism practised by those who had ruled that one year between him and Edwin"), and was the