Cadwallon ap Cadfan was the King of Gwynedd from around 625 until his death in battle. The son and successor of Cadfan ap Iago, he is best remembered as the King of the Britons who invaded and conquered Northumbria, defeating and killing its king, Edwin, prior to his own death in battle against Oswald of Bernicia. His conquest of Northumbria, which he held for a year or two after Edwin died, made him one of the last recorded Celtic Britons to hold substantial territory in eastern Britain until the rise of the Welsh House of Tudor. He was thereafter remembered as a national hero by the Britons and as a tyrant by the Anglo-Saxons of Northumbria.

The Kingdom of Gwynedd was a Welsh kingdom and a Roman Empire successor state that emerged in sub-Roman Britain in the 5th century during the Anglo-Saxon settlement of Britain.