The POD is in AD 1054, when Earl Siward of Northumbria's son Osbjorn does not die in the Battle of the Seven Sleepers. When Siward dies, Northumbria is thus left to his son, who is already a man and quite ready to take on his military and political opponents. Northumbria was IOTL give to Earl Harold Godwinson's brother Tostig, which caused enmity between them and increased the Godwindsons' influence boundlessly. With an allied Northumbria and Mercia, the Godwinsons will find it nigh-impossible to take over England…though who can say how soon childless King Edward the Confessor will die, and what will happen afterward?
Earl Siward of Northumbria, 1000-1023-