The Great Viking Army , known by the Anglo-Saxons as the Great Heathen Army, was a coalition of Norse warriors, originating from Denmark, Norway, Sweden who came together under a unified command to invade the Anglo-Saxon kingdoms that constituted England in AD 865 (816 in the series).
The Great Army, having set sail from Kattegat, arrives on Northumbrian shores. Bishop Unwan and King Aelle set off to meet them immediately, encoutering them close to York. King Aella is convinced his army will rapidly dispose of the Norsemen as he sees what looks like a few dozen warriors. However, thousands of Vikings emerge from behind the hills and regroup, forming a massive and astonishing invading force. Led by Bjorn and Ivar, the Vikings charge and quickly overrun the Northumbrian forces by sheer power (even though the scene isn't shown).
The Great Heathen Army manages to move swiftly and quickly across England, by foot or thanks to their longships with which they can go further inland. The Norsemen soon spread chaos and wreak havoc across the English countryside, especially in Mercia, where many civilians are forced to flee from their advancing foe.The Vikings then disembark near Repton, in the Trent Valley located in Derbyshire. Aethelwulf, King Egbert's son, is informed of their arrival on Mercian soil. Their number is estimated as over 4000 men. Aethelwulf is dispatched with the task of assembling a Saxon army whose goal will be to stop the Norsemen's advance. Aethelwulf assembles a considerable force composed of a few thousand Mercian and Wessex soldiers and immediately moves towards the Vikings positions in ordzer to engage them.
Having been informed by a scout of the progression of the English army, Ivar and Bjorn had already inspected the battlefield and elaborated their strategy. As soon as the English army arrives on the battlefield, the Viking force split in two groups, and Aethelwulf orders his soldiers to follow them. A few hundred Viking warriors then appear behind the pursuing English army, while Bjorn and his force run through the woods in order to trap their foe. The Saxons turn around in order to pursue the fleeing Norsemen but they are met with a rain of arrows coming from the woods. The Saxon suffer important casualties but manage to form a shield wall. The force led by Ivar and Floki suddenly emerge at the top of a hill, and the weakened Saxon pursue them. Ivar then orders Floki to simulate their retreat, which allows the force to run through the woods and surround the English army. Annoyed of being made a fool, Aethelwulf orders his army to go straight for Repton, where the Viking ships are anchored.
However, the Saxons are yet again met with hundreds of arrows pouring down from the top of a hill on which hundreds of Viking archers are inflicting heavy casualties on the English soldiers. The forces led by Bjorn and King Harald then emerge from the top of the hills and form a massive shield wall in order to block the way to Repton. The battle then engages, the forces coliding into each other with great strength, and the hand to hand combat begins, which turns into the Viking's favour. Bjoin gains of King Ecbert, before his suicide, legitimily, lands to etaliate Vikings colonies, honoring the promess done to Ragnar, many years ago before his death.