 While Ælla is described in most sources as a tyrant, and not a rightful king, one source states that he was Osberht's brother.The Great Heathen Army marched on Northumbria in the late summer of 866, seizing York on 21 November 866.Symeon of Durham, the "Anglo-Saxon Chronicle", Asser, and Æthelweard all recount substantially the same version of events in varying detail.