Heimskringla Berserkers

Óðinn could bring it about that in battle his opponents were struck with blindness or deafness or panic, and their weapons would cut no better than sticks, while his men went without mail and were as wild as dogs or wolves, biting their shields, being as strong as bears or bulls. They killed the people, but neither fire nor iron took effect on them. That is called berserk fury.*


- Heimskringla, Ynglinga Saga, Chapter 6


("berserkr (‘bear-shirt’) or ulfheðinn (‘wolf-skin’): a man who fell into, or was able to work himself into, an animal-like state in which his strength was greatly enhanced and he seemed to be invulnerable to weapons. The berserks’ apparently supernatural strength was often said to be bestowed on them by Óðinn." - Footnote included with translation)