Berserkers (or berserks) were Norse warriors who are primarily reported in the Old Norse literature to have fought in a nearly uncontrollable, trance-like fury, a characteristic which later gave rise to the English word berserk. They were fearsome Viking warriors who fought friend and foe whilst in a heightened state of uncontrollable fury – a form of madness known as barsarkargang (going berserk). Berserkers are attested to in numerous Old Norse sources. Most historians believe that berserkers worked themselves into a rage before battle, but some think that they might have consumed drugged foods.
Berserkers appear prominently in a multitude of other sagas and poems, many of which describe berserkers as ravenous men who loot, plunder and kill indiscriminately. Later, by Christian interpreters, the berserk was even viewed as a 'heathen devil'. Snorri Sturluson in Ynglinga Saga, recalling numerous elements of ancient lore, describes Odin's warriors in this way:
'His men went to battle without armor and acted like mad dogs or wolves. They bit into their shields and were as strong as bears or bulls. They killed men, but neither fire nor iron harmed them. This madness is called berserker-fury.'
In medieval Norse and Germanic history and folklore, the berserkers were described as members of an unruly warrior gang that worshipped Odin, the supreme Norse deity, and were commissioned to royal and noble courts as bodyguards and ‘shock troops’, who would strike fear into all who encountered them. Adding to their ferocity, and in order to intimidate the enemy, they would wear bear and wolf pelts when they fought, giving them the name Berserker, meaning 'bear coat' in Old Norse.
The fury of the berserkers would start with chills and teeth chattering and give way to a purpling of the face, as they literally became ‘hot-headed’, and culminating in a great, uncontrollable rage accompanied by grunts and howls. They would bite into their shields and gnaw at their skin before launching into battle, indiscriminately injuring, maiming and killing anything in their path.
Dating back as far as the ninth century, the berserker Norse Warriors were said to be able to do things that normal humans could not. According to ancient legend, the berserkers were indestructible, and no weapon could break them from their trance. They were described as being immune to fire and to the strike of a sword, continuing on their rampage despite injury.
Once the effects of berserkergang had worn off the berserker would lose all his strength and be vulnerable to capture or assassination. After the heat of battle passed, a period of intense exhaustion would follow, a time of vulnerability for the berserkers. As you might guess, despite their prowess in war, the berserks were not loved, for they might turn on friends as well as foes in 'battle madness.'
While some researchers believe the Berserkers simply worked themselves up into a self-induced hysteria before fighting, others maintain that it was sorcery, the consumption of drugs or alcohol, or even mental illness, such as mania or post-traumatic stress, which accounted for their behavior. Some botanists have claimed that berserker behavior could have been caused by the ingestion of the plant known as bog myrtle or mushrooms, one of the main spices in Scandinavian alcoholic beverages.
Other more esoteric theories surround supernatural beliefs. For instance, some scholars have claimed that the Vikings believed in spirit possession and that berserkers were possessed by the animal spirits of wolves or bears. According to some theorists, berserkers learned to cultivate the ability to allow animal spirits to take over their body during a fight (an example of animal totems) that also involved drinking the blood of the animal that they wished to be possessed by.
"Going berserk" is used colloquially to describe a person who is acting in a wild rage or in an uncontrolled and irrational manner. "Berserker" is also a well known character archetype and status in video games and other media.