In the Old Norse written corpus, Berserkers (or “berserks”) were said to have fought in a trance-like fury, a characteristic which later gave rise to the modern English word “berserk.” Berserkers are attested to in numerous Old Norse sources, as were the Úlfhéðnar (“wolf-coats”).
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.
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 shape-shift into a bear’s form.