Of the Viking warriors, the most fierce and powerful were the berserkers. Before battle they would get into a frenzy that allowed them to ignore pain and throw thoughts of survival and safety out the window. They also dressed themselves in bear or wolf skins to make use of the fear common people had for wild animals. They would whip themselves into the battle frenzy by biting their shields and howling like animals. They were ferocious fighters and seemingly unstoppable while this madness lasted. Many tales say the berserker was actually magically immune to weapons. This concept of immunity may have evolved from the berserker's rage, during which the berserk might receive wounds, but due to his state of frenzy take no note of them until the madness passed from him. A warrior who continued fighting while bearing mortal wounds would surely have been a terrifying opponent. The berserker is closely associated in many respects with the god Odin. Legends say that Odin could shape-shift into the form of a bird, fish, or wild animal. The berserker, too, was often said to change into bestial form, or at least to assume the ferocious qualities of the wolf or bear.
Snorri Sturluson wrote in his Ynglinga Saga:
Odin could make his enemies in battle blind, or deaf, or panic-struck, and their weapons so blunt that they could cut no better than a willow-wand; but his own men dashed forward without armour, and became as frenzied as dogs or wolves. They chewed their shield-rims, and became as strong as bears or bulls, and slaughtered people at a single stroke, but neither fire nor iron could touch them. It was called 'going berserk'.
Piece from Lewis chess set depicting a Berserker biting his shield.