On the day before his battle against Cadwallon (or Catlon), King of the Britons, King Oswald of the Northumbrians, had a vision while lying in his tent on the campsite close to the battlefield. In his vision Oswald saw St Columba, the Irish missionary to Northumbria, descending from heaven. The saint appeared to touch the clouds with his head, radiating with the beauty of an ‘angelic form’. The saint spoke to the king, giving him courage and promising him happiness and victory over Cadwallon. Oswald woke up, left his tent and reported the vision to his counsellors. The counsellors heard the report with comfort and relief and promised to convert to Christianity after the battle. Indeed, Oswald won the battle after Cadwallon was killed. He returned home victorious and received divine ordination as the ‘ruler of all Britain’ [totius Brittanniae imperator].

Abbot Adomnán of the Irish monastery at Iona, established by St Columba in 563, began his Life of Columba with this vision. Columba was one of Adomnán's aristocratic relatives, and Adomnán wrote the Life at the very end of the seventh century. He insisted that he had heard of Oswald's vision from his predecessor at Iona, Abbot Failbhe, who in turn reportedly had received news of the dream directly from Oswald. Adomnán introduced Oswald as a ruler over migrants from the continent to whom he referred generically as ‘Saxons’. He added the information that at the time of the battle, Oswald and twelve of his followers, who had been baptised while in exile among the Picts, were the only Christians among the Saxons who were then ‘darkened by the shadow of heathenism and ignorance’.

Bede, who said he completed his Ecclesiastical History of the English People in 731, has his own report on the battle preparations that differs from Adomnán's while confirming some features of Adomnán's version. According to Bede, Oswald set up a cross on the battlefield before the battle began and insisted on holding the cross himself while warriors fixed it in the ground. Oswald sank to his knees after the cross had been placed in the ground, and said a prayer. According to Bede, Oswald and his warriors were in need of divine help in order to triumph over Cadwallon, whom Bede, like Adomnán, styled King of the Britons.