In the Old Norse written corpus, berserkers were those who were said to have fought in a trance-like fury, a characteristic which later gave rise to the modern English word berserk (meaning "furiously violent or out of control"). Berserkers are attested to in numerous Old Norse sources.
The Old Norse form of the word was berserkr (plural berserkir). It likely means "bear-shirt" (compare the Middle English word serk, meaning shirt), "someone who wears a coat made out of a bear's skin". Thirteenth-century historian Snorri Sturluson interpreted the meaning as "bare-shirt", that is to say that the warriors went into battle without armour, but that view has largely been abandoned.
The bas relief carvings on Trajan's column in Rome depict scenes of Trajan's conquest of Dacia in 101–106 AD. The scenes show his Roman soldiers plus auxiliaries and allies from Rome's border regions, including tribal warriors from both sides of the Rhine. There are warriors depicted as barefoot, bare-chested, bearing weapons and helmets that are associated with the Germani. Scene 36 on the column shows some of these warriors standing together, with some wearing bearhoods and some wearing wolfhoods. Nowhere else in history are Germanic bear-warriors and wolf-warriors fighting together recorded until 872 AD with Thórbiörn Hornklofi's description of the battle of Hafrsfjord when they fought together for King Harald Fairhair of Norway.
In the spring of 1870, four cast-bronze dies, the Torslunda plates, were found by Erik Gustaf Pettersson and Anders Petter Nilsson in a cairn on the lands of the farm No 5 Björnhovda in Torslunda parish, Öland, Sweden. Two relevant images are depicted below, along with two associated woodcuts made two years later in 1872.
Torslunda helmet: Odin followed by a berserker
Torslunda helmet: two warriors with boars upon their helmets
It is proposed by some authors that the berserkers drew their power from the bear and were devoted to the bear cult, which was once widespread across the northern hemisphere. The berserkers maintained their religious observances despite their fighting prowess, as the Svarfdæla saga tells of a challenge to single-combat that was postponed by a berserker until three days after Yule. The bodies of dead berserkers were laid out in bearskins prior to their funeral rites. The bear-warrior symbolism survives to this day in the form of the bearskin caps worn by the guards of the Danish monarchs.
In battle, the berserkers were subject to fits of frenzy. They would howl like wild beasts, foam at the mouth, and gnaw the iron rim of their shields. According to belief, during these fits they were immune to steel and fire, and made great havoc in the ranks of the enemy. When the fever abated they were weak and tame. Accounts can be found in the sagas.[better source needed]
To "go berserk" was to "hamask", which translates as "change form", in this case, as with the sense "enter a state of wild fury". Some scholars have interpreted those who could transform as a berserker was typically as "hamrammr" or "shapestrong" – literally able to shapeshift into a bear's form.:126 For example, the band of men who go with Skallagrim in Egil's Saga to see King Harald about his brother Thorolf's murder are described as "the hardest of men, with a touch of the uncanny about a number of them ... they [were] built and shaped more like trolls than human beings." This has sometimes been interpreted as the band of men being "hamrammr", though there is no major consensus. Another example of "hamrammr" comes from the Saga of Hrólf Kraki. One tale within tells the story of Bödvar Bjarki, a berserker who is able to shapeshift into a bear and uses this ability to fight for king Hrólfr Kraki. "Men saw that a great bear went before King Hrolf's men, keeping always near the king. He slew more men with his fore paws than any five of the king's champions."
Wolf warriors appear among the legends of the Indo-Europeans, Turks, Mongols, and Native American cultures. The Germanic wolf-warriors have left their trace through shields and standards that were captured by the Romans and displayed in the armilustrium in Rome. 
The frenzy warriors wearing the skins of wolves were called Úlfhéðnar ("wolf coat"; singular Úlfheðinn), another term associated with berserkers, mentioned in the Vatnsdæla saga, the Haraldskvæði and the Völsunga saga. They 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'.":132 In addition, the helm-plate press from Torslunda depicts a scene of Odin with a berserker with a wolf pelt and a spear as distinguishing features: “a wolf skinned warrior with the apparently one-eyed dancer in the bird-horned helm, which is generally interpreted as showing a scene indicative of a relationship between berserkgang ... and the god Odin”.
Berserkers appear prominently in a multitude of other sagas and poems. Many earlier sagas portrayed berserkers as bodyguards, elite soldiers, and champions of kings. This image would change as time passed and sagas would begin to describe berserkers as boasters rather than heroes, and as ravenous men who loot, plunder, and kill indiscriminately. Within the sagas, Berserkers can be narrowed down to four different types. The King's Berserkr, the Hall-Challenging Berserkr, the Hólmgangumaðr, and the Viking Berserkr. Later, by Christian interpreters, the berserker was 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 honor of King Harald Fairhair, as ulfheðnar ("men clad in wolf skins"). This translation from the Haraldskvæði saga describes Harald's berserkers:
I'll ask of the berserks, you tasters of blood,
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.
The "tasters of blood" in this passage are thought to be ravens, which feasted on the slain.
His (Odin's) men rushed forwards without armour, were as mad as dogs or wolves, bit their shields, and were strong as bears or wild oxen, and killed people at a blow, but neither fire nor iron told upon them. This was called Berserkergang.
King Harald Fairhair's use of berserkers as "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 Männerbünde, or used the name as a deterrent or claim of their ferocity.
Emphasis has been placed on the frenzied nature of the berserkers, hence the modern sense of the word "berserk". However, the sources describe several other characteristics that have been ignored or neglected by modern commentators. Snorri's assertion that "neither fire nor iron told upon them" is reiterated time after time. The sources frequently state that neither edged weapons nor fire affected the berserks, although they were not immune to clubs or other blunt instruments. For example:
These men asked Halfdan to attack Hardbeen and his champions man by man; and he not only promised to fight, but assured himself the victory with most confident words. When Hardbeen heard this, a demoniacal frenzy suddenly took him; he furiously bit and devoured the edges of his shield; he kept gulping down fiery coals; he snatched live embers in his mouth and let them pass down into his entrails; he rushed through the perils of crackling fires; and at last, when he had raved through every sort of madness, he turned his sword with raging hand against the hearts of six of his champions. It is doubtful whether this madness came from thirst for battle or natural ferocity. Then with the remaining band of his champions he attacked Halfdan, who crushed him with a hammer of wondrous size, so that he lost both victory and life; paying the penalty both to Halfdan, whom he had challenged, and to the kings whose offspring he had violently ravished...
Similarly, Hrolf Kraki's champions refuse to retreat "from fire or iron". Another frequent motif refers to berserkers blunting their enemy's blades with spells or a glance from their evil eyes. This appears as early as Beowulf where it is a characteristic attributed to Grendel. Both the fire eating and the immunity to edged weapons are reminiscent of tricks popularly ascribed to fakirs.
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.
Scholar Hilda Ellis-Davidson draws a parallel between berserkers and the mention by the Byzantine emperor Constantine VII (AD 905–959) in his book De cerimoniis aulae byzantinae ("Book of Ceremonies of the Byzantine court") of a "Gothic Dance" performed by members of his Varangian Guard (Norse warriors in the service of the Byzantine Empire), who took part wearing animal skins and masks: she believes this may have been connected with berserker rites.
The rage the berserker experienced was referred to as berserkergang (Berserk Fit/Frenzy or The Berserk movement). This condition has been described as follows:
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 colour. 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.
When Viking villages went to war in unison, the berserkers often wore special clothing, for instance furs of a wolf or bear, to indicate that this person was a berserker, and would not be able to tell friend from foe when in "bersærkergang". In this way, other allies would know to keep their distance.
Some scholars propose that certain examples of berserker rage had been induced voluntarily by the consumption of drugs such as the hallucinogenic mushroom Amanita muscaria or massive amounts of alcohol. However, this is much debated and has been thrown into doubt by the discovery of seeds belonging to the plant henbane Hyoscyamus niger in a Viking grave that was unearthed near Fyrkat, Denmark in 1977. An analysis of the symptoms caused by Hyoscyamus niger are also similar to the symptoms ascribed to the berserker state, which suggest it may have been used to generate their warlike mood. Other explanations for the berserker's madness that have been put forward include self-induced hysteria, epilepsy, or mental illness, among other causes.
One theory of the berserkers suggests that the physical manifestations of the berserker alongside their rage was a form of self-induced hysteria. Initiated before battle through a ritualistic process, also known as effektnummer, which included actions such as shield-biting and animalistic howling.
If a soldier survives the berserk state, it imparts emotional deadness and vulnerability to explosive rage to his psychology and permanent hyperarousal to his physiology — hallmarks of post-traumatic stress disorder in combat veterans. My clinical experience with Vietnam combat veterans prompts me to place the berserk state at the heart of their most severe psychological and psychophysiological injuries.
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