Heimskringla Abilities

When Óðinn of the Æsir came to the northern lands, and the gods with him,
it is truthfully said that they originated and taught those skills that people
went on practising long afterwards. Óðinn was superior to them all, and from
him they learned all skills, because he knew them all first, and yet more.
But there is this to be said about why he was so very exalted—there were
these reasons for it: he was so fair and noble in countenance, when he was
sitting among his friends, that it rejoiced the hearts of all. But when he went
to battle he appeared ferocious to his enemies. And the reason was that he
had the faculty of changing complexion and form in whatever manner he
chose. Another was that he spoke so eloquently and smoothly that everyone
who heard thought that only what he said was true. Everything he said was
in rhyme, like the way what is now called poetry is composed. He and
his temple priests were called craftsmen of poems, for that art originated
with them in the Northern lands. 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)