The earliest surviving reference to the term berserker is in Haraldskvæði, a skaldic poem written by Thórbiörn Hornklofi in the late ninth century in honour of King Harald Fairhair, the famous ruler of Norway. The poem was preserved by Snorri Sturluson. In this poem, Harald's army includes a warrior gang of berserkers fighting under his name at the battle of Hafrsfiord. In it, they are described as Ulfheðnar = "men clad in wolf skins". This grounds a connection between bears and wolves in Norse warrior culture and the common assumption that the word "berserker" itself originates from men wearing the skin of the bear. Snorri Sturluson goes on to mention berserkers in the Ynglinga saga: "his [Odin's] men rushed forward without armor, were as mad as dogs or wolves, bit their shields, and were as strong as bears or wild bulls, and killed people at a blow, but neither fire nor iron told upon themselves" (Ch. 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.
Berzerkers were Odin's Warriors. As men who could transform into the spirit of wolves or bears, aquiring their strength and/or ferocity, they were feared greatly by the more civilized peoples who lived in or near Europe before the 1100's. They were known to savage and destroy friend and foe alike. Women and children had to be kept far from them. They were, in a word, fearsome.
In creating a World in which I have an area of the world devoted to Norse myth I am exploring Berzerkers from the historical and mythological perspective. It appears they were a form of Shaman who, at the height of their powers, could commune with the Otherworld King, Odin, and transform themselves into wolves or bears. There is some mention of them being able to do so even physically, and so there is a question as to whether or not Berzerkers are to be linked to Lycanthropes (werewolves). At any rate they were deeply spiritual (though not in the flower-lovey-dovey way people use that word currently) and were principally believed to be possessed by the animal spirits of the wolf or bear. What does this imply for World Weaving a land in which Berzerkers thrive? What would that region be like? For this my sources are 'Beowulf', and the Islandic Sagas such as 'Egil's Saga', as well as 'The Nibilungenlied' and 'The Kalevala'.
It was a wild land of lawless warfare and high magic. Fascinating indeed.