On August 5, 642, the pagan King Penda of Mercia amassed his own forces, together with the Welsh armies of Gwynedd, Powys and Pengwern, against Oswald, the Christian King of Northumbria. They met at the town of Maserfield in modern day Shropshire, where Oswald was killed. In the Venerable Bede's Ecclesiastical History of the English People, it is recorded that Oswald died a martyr's death and "ended his life in prayer" for the souls of his soldiers, before he was decapitated by an axe and his body was dismembered by Penda's men.
Legend has it that, as a warning to others, Oswald's limbs were hung from the branches of an ash tree by his victors, and that a holy well sprang up at the spot where his arm fell from the tree. Variations suggest that one of his arms was carried to the tree by an eagle, where it was dropped to the ground below.
The town of Oswestry ('Oswald's Tree') has stood on the site of Maserfield since at least 1190. The site of Oswald's Well marks the spot where the spring is supposed to have originated. Oswald was made a saint for converting the people of Northumbria to christianity and his remains are said to have been removed to St Oswald's Priory in Gloucestershire in 909 by Lady Aethelflaed (the formidable wife of Aethelred II, the last king of Mercia), who gathered together St Oswald's various relics so that they may be interred in one site.