The Úlfhéðnar (singular Úlfheðinn), another term associated with berserkers, mentioned in the Vatnsdœla saga, Haraldskvæði and the Völsunga saga, were said to wear the pelt of a wolf when they entered battle. Úlfhéðnar are sometimes described as Odin's special warriors:
"[Odin’s] men went without their mailcoats and were mad as hounds or wolves, bit their shields...they slew men, but neither fire nor iron had effect upon them. This is called 'going berserk."
In addition, a helm-plate press from Torslunda depicts a scene of Odin with a berserker—"a wolf skinned warrior with the dancer in the bird-horned helm, which is generally interpreted as showing a scene indicative of a relationship between berserkgang... and the god Odin" — with a wolf pelt and a spear as distinguishing features.
To "go berserk" can be also considered to “hamask”, which can be translated as “change form," in this case, as with the sense "enter a state of wild fury" and one who could transform as a berserker was typically thought of as “hamrammr” or “shapestrong”.
Berserkers appear prominently in a multitude of other sagas and poems, many of which describe them as ravenous men who loot, plunder, and kill indiscriminately. Later, by Christian interpreters, the berserk was even viewed as a "heathen devil."
The earliest surviving reference to the term "berserker" is in Haraldskvæði, a skaldic poem composed by Thórbiörn Hornklofi in the late 9th century in honour of King Harald Fairhair, as ulfheðnar ("men clad in wolf skins"). This translation from the Haraldskvæði saga describes Harald's berserkers:
Those intrepid heroes, how are they treated,
Those who wade out into battle?
Wolf-skinned they are called. In battle
They bear bloody shields.
Red with blood are their spears when they come to fight.
They form a closed group.
The prince in his wisdom puts trust in such men
Who hack through enemy shields."
In 1015, Jarl Eiríkr Hákonarson of Norway outlawed berserkers. Grágás, the medieval Icelandic law code, sentenced berserker warriors to outlawry. By the 12th century, organised berserker war-bands had disappeared.
Nowadays, the word "berserker" applies to anyone who fights with reckless abandon and disregard to even his own life, a concept used during the Vietnam War and in Vietnam-inspired literature and film. "Going berserk" in this context refers to a state induced by adrenaline (or military-issued amphetamine for long missions) in the human body and brain leading a soldier to fight with fearless rage and indifference, a state strikingly similar to that of the 9th century berserkers. "Going berserk" is also used colloquially to describe a person who is acting in a wild rage or in an uncontrolled and irrational manner. Furthermore, "berserker" is also a well known character archetype and status in video games and other media.