After his victory, Athelferth advanced as far as the Firth of Forth, but was unable to hold the territory he had overrun. With no threats from the north (the Britons of the (albeit powerful) kingdom of Strathclyde deemed it wiser not to challenge him), he now turned his attention elsewhere.
In 604, he occupied York and conquered Deira. Edwin (whose father Aelle had taken the city from Gwrgi and Peredur Steel-Arm), was forced to flee to Gwynedd. (It is important to understand that through this turbulent period, sides were not drawn along the lines of either ethnicity or religion.) The son of Edwin's nephew, Hereric, fled to the Welsh kingdom of Elmet around Leeds. Aethelferth ruled without warring until 614 when he invaded north Wales, perhaps a pre-emptive strike caused by concern that the Britons sheltering Edwin would help him try to regain Deira.
Aethelferth's army marched to Chester where it met the armies of Gwynedd, Powys and the Cornovii from what is now Staffordshire. Before the battle began, a thousand monks from Bangor-on-Dee, who had accompanied the Welsh armies, began to pray for victory. This must have been a great boost for the morale of the Britons. How much greater then that blow to their morale, when the Heathen Aethelferth on observing this, said "If these men invoke their god against us, they fight against us, even if they have no arms."
He then promptly launched his first attack against the defenceless monks, and slaughtered them all before going on to win the battle. This victory now separated, by land, the Britons of the far north, from those in Wales. (Just as the British loss of Gloucester twenty-five years earlier had made overland military movement between Wales and Dumnonia (Devon) impossible.)
By this time, Edwin had sought refuge with Raedwald, the king of East Anglia. (The famous ship burial at Sutton Hoo was probably Raewald's memorial, though intriguingly it contained no body; it is known that Raedwald did not give up the worship of his ancestral gods even after he began to worship the Christian god; perhaps he had a Christian burial and his Heathen, nominally Christian, subjects saw to it that a fully equipped ship was prepared to take his soul to the otherworld.) Aethelferth demanded that Raedwald hand Edwin over to him, but Raedwald took heed of his wife's view that such a betrayal of a guest would not be honourable, and refused. Their armies then fought in 616, where the Roman road between Lincoln and Doncaster crosses the River Idle; Aethelferth died in this battle and his sons fled in exile.
Edwin now became king of Northumbria and accepted baptism. He conquered Elmet, killing its king Ceredic for poisoning Edwin's nephew's son, Hereric. Edwin later suffered an invasion of Northumbria by the combined armies of Gwynedd under Catwallaun and Penda of Mercia (the last English king to die a Heathen). In 632, Edwin and his son were killed in battle at Hatfield Chase. By 633, Oswald, Aethelferth's son, returning suddenly from exile in Ireland, defeated Catwallaun at Rowley Burn, south of Hexham, and made himself king of Northumbria and became overlord of all the kingdoms south of the Humber.