They are also sometimes called Ulfheônar ("Wolf-skin-clad Ones").
In old-Norse sagas, they were warriors who dressed themselves in bear skins, to make use of the fear common people had for wild animals. They whipped themselves up to a sort of battle frenzy, biting their shields and howling like animals. They were ferocious fighters and seemingly insensitive to pain while this madness lasted; berserks made formidable enemies. In their rage they even attacked the boulders and trees of the forest; it was not uncommon that they killed their own people. The belief in berserks can be compared with the belief in werewolves; both are magical transformations of humans who assume the shape of an kindred animal.