866/7. "Great Heathen Army" moves from East Anglia to Northumbria, takes York
The Anglo-Saxon Chronicle records that at the time of the arrival of the army in Northumbrian York there was great civil strife among that people, who had deposed their king Osberht and taken a king with no hereditary right, Ælle. It was not until late in the year that they united sufficiently to fight against the raiding army. They gathered a large army and attacked the Vikings who held York, but there was an immense slaughter of the Northumbrians, and both kings were killed, and the survivors made peace with the enemy.
Later sources give more details, though of uncertain value: the 12th-century History of the Church of Durham notes that the Vikings took York on November 1, 867 (866 in our reckoning), and that the English tried to take back York on March 21, 867. Simeon of Durham adds in his Historia Regum that after that the Vikings set up Ecgberht as ruler under their dominion of the Northumbrians beyond the Tyne, which was presuambly roughly the same as the ancient kingdom of Bernicia. Simeon adds that Ecgberht died in 873 and was succeeded by Ricsige, and that Ricsige was succeeded in 876 by Ecgberht the second. However, effective power will have been held by the Vikings, who visited in 872, wintered by the Tyne in 874/5, and shared out the land of the Northumbrians in 876.