What’s the Great Heathen Army? I’ve never seen that particular theory floated around before.

The Great Heathen Army was a real-world historical event, when the sons of Ragnar Lothbrok gathered together a substantial force of vikingers who would not raid but conquer. The Great Heathen Army, as the Saxons named it, landed in East Anglia in 865 CE, took York in the following year, and then proceeded to seize large swathes of eastern England (East Anglia, Northumbria, eastern Mercia) and almost succeeded in overrunning Wessex.