Berserkers (or berserks) were a special kind of Norse warrior who were widely feared. 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 battle. Others think they might have eaten drugged foods. The Ynglinga saga said they charged their enemy without any armour. 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.