Berserker

Berserkers (or berserks) were a special kind of Norse warrior who were widely feared.James A. Corrick, The Rough, Stormy Age of Vikings (Mankato, MN: Capstone Press, 2011), p. 12  Old Norse literature said they fought in a wild, trance-like fury. This later gave us the English word berserk. 

Many Old Norse sources say that berserkers were real. Most historians believe that berserkers worked themselves into a rage before the battle. Others think they might have eaten drugged foods. The Ynglinga saga said they charged their enemy without any armour.René Chartrand; et al, The Vikings: Voyagers of Discovery and Plunder (Oxford; New York: Osprey, 2008), p. 102 Also that they were as mad as wolves. They could kill the enemy with a single blow of their sword while neither fire nor iron could hurt them.

 References 

Category:Norse mythology