His hatred of the Anglo-Saxon kingdom of Northumbria intensified when his attempts at invasion (629) failed and he was forced to flee to Ireland. Although a Christian, he next allied with the heathen King Penda of Mercia. Their victory at Hatfield Chase (632) over Edwin of Northumbria was followed by the devastation of Northumbria. Thereafter Northumbrian fortunes recovered and Cadwallon was killed in battle by Edwin’s nephew Oswald at Heavenfield, near Hexham.