Wikipedia:
Berserkers (or berserks) were Norse warriors who are reported in the Old Norse literature to have fought in a nearly uncontrollable, trance-like fury, a characteristic which later gave rise to the English word berserk. Berserkers are attested to in numerous Old Norse sources. Most historians believe that berserkers worked themselves into a rage before battle, but some think that they might have consumed drugged foods.
Below is an excerpt from Mircea Eliade's 1958 book,
"Rites And Symbols of Initiation: The Mysteries of Birth and Rebirth." Harper & Row, Publishers: New York. Pg. 81-84.
In a passage that has become famous, the Ynglingasaga sets the comrades of Odin before us: "They went without shields, and were mad as dogs or wolves, and bit on their shields, and were as strong as bears or bulls; men they slew, and neither fire nor steel would deal with them; and this is what is called the fury of the berserker." This mythological picture has been rightly identified as a description of real men's societies---the famous Männerbünde of the ancient Germanic civilization. The berserkers were, literally, the "warriors in shirts (serkr) of bear." This is as much as to say that they were magically identified with the bear. In addition they could sometimes change themselves into wolves and bears. A man became a berserker as the result of an initiation that included specifically martial ordeals. So, for example, Tacitus tells us that among the Chatti the candidate cut neither his hair nor his beard until he had killed an enemy. Among the Taifali, the youth had to bring down a boar or a wolf; among the Heruli, he had to fight unarmed. Through these ordeals, the candidate took to himself a wild-animal mode of being; he became a dreaded warrior in the measure in which he behaved like a beast of prey. He metamorphosed himself into a superman because he succeeded in assimilating the magicoreligious force proper to the carnivora.
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The martial ordeal par excellence was the single combat, conducted in such a way that it finally roused the candidate to the "fury of the berserkers." For not military prowess alone was involved. A youth did not become a berserker simply through courage, physical strength, endurance, but as the result of a magicoreligious experience that radically changed his mode of being. The young warrior must transmute his humanity by a fit of aggressive and terror-striking fury, which assimilated him to the raging beast of prey. He became "heated" to an extreme degree, flooded by a mysterious, nonhuman, and irresistible force that his fighting effort and vigor summoned from the utmost depths of his being. The ancient Germans called this sacred force wut, a term that Adam von Bremen translated by furor; it was a sort of demonic frenzy, which filled the warrior's adversary with terror and finally paralyzed him. The Irish ferg (literally "anger"), the homeric menos, are almost exact equivalents of this same terrifying sacred experience peculiar to heroic combats. J. Vendryes and Marie-Louise Sjoestedt have shown that certain names applied to the Hero in Old Irish refer to "ardor, excitation, turgescence." As Miss Sjoestedt writes, "The Hero is the man in fury, possessed by his own tumultuous and burning energy."
