Were Viking berserkers real, and what made them fight in a frenzy?

Berserkers were real in the sources, but what they really were is debated. Old Norse texts describe Scandinavian warriors who fought in a trance-like fury, which gave English the word berserk. In battle they were subject to fits of frenzy: howling like wild beasts, foaming at the mouth and gnawing the rims of their shields, and believed to be immune to steel and fire.

The name is assumed to mean 'bear-shirt', a coat of bearskin. The 13th-century historian Snorri Sturluson read it as 'bare-shirt', warriors fighting without armour, but that view has largely been abandoned for lack of supporting evidence. The earliest surviving reference is a late-9th-century poem for King Harald Fairhair, which calls them ulfheðnar, 'men clad in wolf skins'.

Then the image soured. Earlier sagas portray berserkers as bodyguards, elite soldiers and champions of kings; later ones describe boasters and ravenous men who loot, plunder and kill indiscriminately. Christian interpreters viewed the berserker as a 'heathen devil'. In 1015 the Norwegian jarl Eiríkr Hákonarson outlawed them, the Icelandic law code Grágás sentenced them to outlawry, and organised berserker war-bands had disappeared by the 12th century.

What caused the frenzy? Nobody knows. Some scholars propose drugs such as hallucinogenic mushrooms or massive amounts of alcohol, and black henbane seeds found in a Viking grave near Fyrkat in Denmark in 1977 are said to support the idea, but it is much debated. Another theory is a self-induced hysteria brought on by ritual before battle. The Lewis Chessmen, thought to be of Norse manufacture, include berserkers biting their shields.