They were the most fearsome shock troops Europe had ever known.
From the late eighth to the early eleventh century, most Europeans lived in fear of the Vikings who could strike at any point along Europe's coastlines in their warships. Although the Viking reputation for fierceness was legendary, the most terrifying of them all were the berserkers.
The actual origin of the word is open to dispute and may have referred to the hair shirts that the berserkers wore into battle (the term literally means "bear fur shirt"). They were a very special breed of warriors who became gripped with "berserker rage" and often plunged into battle without fear or armour. Known as bärsärkar-gångbb ("berserker-path"), the fit of madness that descended on berserkers caused them to be capable of almost superhuman strength and endurance. In addition to using swords, berserkers were also famed for ripping into their opponents with their teeth and fingernails. Their custom of wearing wolf or bearskins fed into many of the superstitions of the time and gave them the reputation of being werewolves or warriors possessed by an animal's spirit. Some authorities suggested that berserkers were part of a cult dedicated to the god Odin (they were commonly known as the "chosen ones of Odin") and added to their fierce image by dying their bodies, drinking wolf or bear blood, and wearing animal masks.
Descriptions of bärsärkar-gång tend to be unreliable since there were so few eyewitness accounts. According to one fascinating article on bärsärkar-gång, "This fury, which was called berserkergang, occurred not only in the heat of battle, but also during laborious work. Men who were thus seized performed things which otherwise seemed impossible for human power. This condition is said to have begun with shivering, chattering of the teeth, and chill in the body, and then the face swelled and changed its color. With this was connected a great hot-headedness, which at last gave over into a great rage, under which they howled as wild animals, bit the edge of their shields, and cut down everything they met without discriminating between friend or foe. When this condition ceased, a great dulling of the mind and feebleness followed, which could last for one or several days". Suffice it to say, berserkers were not fun to be around.
Berserker war bands seemed unstoppable and became the basis of numerous sagas and legends (which is why"going berserk" is still part of our language). Just about every Viking king had his own group of berserker bodyguards and used berserker gangs to expand territory. Berserker rage was hardly limited to the battlefield however. Vikings who were susceptible to barärsärkar-gång often became uncontrollable in any situations that caused them to lose their tempers. Viking warriors who committed murder or rape while affected by berserker rage were executed or exiled. By the eleventh century, even Viking societies had enough of their special warriors and the berserkers were banned from Iceland and Norway. Organized berserker bands were eventually outlawed and barärsärkar-gång became virtually unknown by 1200.;
Later sagas and legends often tended to portray berserkers as being superhuman and, at times, even supernatural. Descriptions of their ferocious nature were likely exaggerations (there were no real berserkers for comparison). One 13th century saga described a berserker named Otrygg who "feared neither fire nor sword" but who was killed after a Christian warrior struck him with a crucifix before cutting off his arm with a sword. The Ynglinga Saga told of berserkers who "went into battle without armor, like mad dogs or wolves, biting their shields, strong like bears or bulls, mowing down everything in their path, immune to fire and iron". Propaganda or not, it certainly made a good story.
There have been different theories relating to bärsärkar-gång and what triggered the incredible berserker rage. The most commonly accepted theory relates to the use of the fly agaric mushroom (amanita muscaria). According to one source, the mushrooms would be fed to sacred reindeer. After the reindeer metabolized the mushrooms, the urine became a powerful natural amphetamine which the berserkers would drink just before going into battle (don't try this at home kids). Modern researchers examining amanita muscaria poisoning have reported episodes of irrational violence, profuse sweating, disorientation, vertigo, and a subsequent "crash" very similar to accounts of berserker range. Other psychoactive substances that have been suggested include bog myrtle, bufotenin, or just plain alcohol. Although explanations such as epilepsy, hysteria, or genetic predisposition to violence have also been suggested, the lack of any modern berserkers for comparison makes the question academic.
It's tempting to suggest that bärsärkar-gång may have been a culture-specific psychosis similar to amok syndrome but, again, we'll never know for sure. While there is a recognized psychiatric condition known as episodic dyscontrol syndrome characterized by episodes of impulsive violence and irrational behaviour, there's no way to link it to bärsärkar-gång. It's ironic that countries populated by descendants of the same berserkers who once raged across Europe now have some of the lowest rates of violence in the world. Whether their ancestors would approve or not is another story.