The Sack of Lindisfarne occurred in 793 AD when a Viking warband under Ragnarr Lodbrok raided the Northumbrian monastery at Lindisfarne in the British Isles, ushering in the "Viking Age". The Vikings were surprised to see such great quantities of treasure unprotected and left in the open, motivating them to return to Britannia time and again.
Background[]
During the late 8th century AD, the Norwegian Vikings of Kattegat, led by Earl Haraldsson, launched yearly raids against their equally-impoverished neighbors in the Baltics and Russia, bringing home small amounts of loot for their lord. Haraldsson was in complete control of the selection of raid targets, as he owned all of the ships in Kattegat, and he refused to risk the loss of any of his ships on the ocean tides outside of the Baltic Sea. However, the free-spirited young raider Ragnarr Lodbrok secretly plotted to head west and seek greater riches and new lands across the North Sea. With the help of Hrafna-Floki Vilgertharson, he built a sturdy longship and, with the help of his brother Rollo Sigurdsson and his friend Eric Marteinn, he assembled a raiding warband. They set sail without informing the Earl, and, using a makeshift water-board compass, they navigated their way west and braved the ocean until they sighted land at the island of Lindisfarne off the Northumbrian coast.
Sack[]

The Vikings entering Lindisfarne
The people of Northumbria had already been terrified by excessive whirlwinds, lightning, and the supposed sight of fiery dragons in the sky, with Brother Athelstan of the monastery interpreting it as foreboding the day of judgment prophesied by Jeremiah. The day after the storm cleared, the Viking longship emerged from the fog, frightening a monk who had been gathering food on the beach. The monks rang their alarm bells, inadvertently letting the Vikings know that they were expecting an attack. The monks then locked themselves into the monastery and frantically prayed, and the Vikings easily broke into the monastery and then entered the room with the monks. Upon seeing the elderly prior start praying out loud, Rollo slit his throat, inaugurating a terrible slaughter; Ragnarr, Eric, and Leif Marteinn were the only three Vikings not to kill any priests. The three men then found the treasure room unguarded, amazed at the fact that the whole monastery was theirs for the taking without opposition. They then took Athelstan prisoner after discovering that he spoke Norse and could be a useful servant, and the Vikings proceeded to thoroughly loot the monastery before leaving with a few captive priests.
Aftermath[]

The Vikings leaving Lindisfarne
After Ragnarr and his men returned to Kattegat with considerable loot, Earl Haraldson reluctantly gave his permission for Ragnarr to launch further raids on Britannia. This ushered in the age of Viking expansion, as the Norse raiders were no longer confined to their Baltic hunting grounds, and were now inspired to launch further raids on monasteries, villages, and towns in Britannia and Western Europe as well.