Berserkers (or Berserks) were Norse warriors who wore coats of wolf or bear skin and who were commonly understood to have fought in an uncontrollable rage or trance of fury, hence the modern word berserk.
The Úlfhéðnar (singular Úlfhéðinn) mentioned in the Vatnsdœla saga, Haraldskvæði and the Völsunga saga were said to wear the pelt of a wolf upon their heads when they entered battle. (For example: Bernhari, Haimric, Hlodwig, Theudberga, Warinhari, etc.) Úlfhéðnar are sometimes described as Odin's special warriors, with the pelt from a wolf and a spear as distinguishing feature.
Snorri Sturluson goes on to mention berserkers in the Ynglinga saga (chapter 6):
Berserkers appear prominently in a multitude of other sagas and poems including The Saga of Hrólf Kraki, many of which describe berserkers as ravenous barbarians who loot, plunder, and kill indiscriminately. They also wore bear coats.
Much can be derived about berserkers from Egils saga. Egil's grandfather was named Kveld-Ulf meaning "evening wolf", and this is generally ascribed as meaning he was a werewolf. Kveld-Ulf's son, referred to as Skalla-Grimm, was a berserker. Kveld-Ulf and Skalla-Grimm are both depicted as irascible and violent throughout the saga, the latter attempting to kill his son. Egill Skallagrímsson himself is described in the saga as attacking opponents with his teeth, ripping out another berserker's jugular vein during a duel. Patently, violence and gruesome tragedies permeate the berserker ethos described in Icelandic sagas such as this one.
In 1015 Jarl Eiríkr Hákonarson of Norway outlawed berserkers. Grágás, the medieval Icelandic law-code, sentenced berserker warriors to outlawry. By the 1100s, organized berserker warbands had disappeared.
King Harald Fairhair's use of berserker "shock troops" broadened his sphere of influence. Other Scandinavian kings used berserkers as part of their army of hirdmen and sometimes ranked them as equivalent to a royal bodyguard. It may be that some of those warriors only adopted the organization or rituals of berserk warbands or used the name as a deterrent or claim of their ferocity.
Still, some scholars consider the frenzied and indomitable berserker and his bloodshot eyes to stand right alongside horned Viking helmets as a "feature of later literary [works] rather than contemporary historical ones", placing the legitimacy of Icelandic sagas as historical records into question. Little Icelandic literature was recorded before the mid-thirteenth century, more than two hundred years after the conversion of Iceland to Christianity. The sagas are broadly interested in history, but they are re-tellings of legend and in no way constitute a proper historical record. The family sagas in particular shed more light on 13th and 14th-century ideas about the 9th-11th centuries than they do on the legendary period itself.
A Horizon Book on Vikings claims that some chieftains would hold their berserkers in reserve during a battle. Once a portion of the enemy line appeared to tire or weaken, the chieftains would send the berserkers charging into the enemy ranks to hopefully open a break and even panic the enemy. The book also claimed that while on sea voyages close to land, berserkers were sometimes asked to go ashore to find objects on land to wrestle or bash to give vent to their fury.
According to a theory of spirit possession, the berserk rage was achieved through possession by the animal spirit of either a bear or a wolf. Berserkers would cultivate an ability to allow the animal's spirit to take over their body during a fight. This is seen as a somewhat peculiar application of animal totemism.
Botanists have suggested the behavior might be tied to ingestion of bog myrtle (Myrica gale syn: Gale palustris), a plant that was one of the main spices in alcoholic beverages in Scandinavia. The drawback is that it increases the hangover headache afterwards. Drinking alcoholic beverages spiced with bog myrtle the night before going to battle might have resulted in unusually aggressive behavior.
The notion that Nordic Vikings used the fly agaric mushroom to produce their berserker rages was first suggested by the Swedish professor Samuel Ödman in 1784. Ödman based his theory on reports about the use of fly agaric among Siberian shamans. The notion has become widespread since the 19th century, but no contemporary sources mention this use or anything similar in their description of berserkers. In addition, the injection of bufotenine from Bufo marinus toad skin into humans was shown to produce similar symptoms to the "Berserker" descriptions. These findings, first examined by Howard Fabing in 1956, were later linked to the induction of zombie characteristics by ethnobotanists in 1983.
A British television program in 2004 tested the possible use of fly agaric and alcohol by training a healthy volunteer in the use of Viking weapons, then evaluating his performance under their influence. It was shown that use of fly agaric or alcohol severely reduced his fighting ability, and the tentative conclusion was that the berserk state was achieved psychologically; otherwise, berserkers would have been too easy to kill. Of course, this does not take into account the mindset that the berserker likely would have attempted to place himself in.
A simpler theory attributes the behavior to drunken rage. It is also possible that berserkers worked themselves into their frenzy through purely psychological processes, perhaps using frenzied rituals and dances. According to Saxo Grammaticus they also drank bear or wolf blood.
American professor Jesse L. Byock claims (in Scientific American, 1995) that berserker rage could have been a symptom of Paget's disease. Uncontrolled skull bone growth could have caused painful pressure in the head. He mentions the unattractive and large head of Egill Skallagrímsson in Egilssaga. Other possibilities are mild epilepsy, rabies, and hysteria. Nevertheless, these theories are highly unlikely, as the berserkers would presumably turn on each other as well as their enemies. During battle, they are consistently described in the frenzy of rage; yet berserkers, while purportedly felling allies, seem to have avoided attacking each other.
Similar behaviour is described in the Iliad, where warriors who are "possessed" by a god or goddess exhibit superhuman powers.
In historical times, the Spartan warrior Aristodemus in mentioned as acting with a berserker-like fury at the Battle of Plataea, to redeem himself from accusations of having acted with cowardice at Thermopylae.
The Bible compares King David's "bitter warriors", who fight with such fury that they could overcome many times their number of opponents, with "a bear robbed of her whelps in the field" (2 Book of Samuel, 17, 8 - see ).
Modern soldiers in battle sometimes observe in both themselves and others these occasional bouts of exceptional aggressiveness and feelings of invincibility. The main character in the Red Badge of Courage has a moment in battle where he becomes almost unaware of the danger around him and experiences an irresistible urge to destroy the enemy. Afterwards, he comes back to himself and is only vaguely aware of what has happened. The testimony of Medal of Honor recipients and other combat veterans sometimes recount similar experiences of altered consciousness and heightened aggression during combat . There is however, no real modern military tradition or documentation of the habitual inducement of a 'berserker' mental state. Since modern combat training focuses on intensive drilling, military specialties, and unit cohesion, unpredictable and individualistic 'berserker' combat is most certainly discouraged in modern military science and training.
"Going berserk" is also used colloquially to describe a person who is acting in a wild rage or in an uncontrolled and irrational manner. A recent controversy among civil rights advocates and law enforcement and emergency medicine professionals involves a state called "excited delirium", in which a "berserk" individual dies after the use of restraints.
The anime series "Berserk" written and drawn by Kentaro Miura depicts a character who repeatedly loses himself in his fury, sometimes for years at a time. The work, though covering many themes and spanning twenty years of manga-series releases, nevertheless centers on the one character, Guts, and what pushes him into these fits of trance-like fury.