Bersekergang

Berserkergang is the name for that sacred state of consciousness, said to be the rather ambiguous gift of the god Odhinn to some warriors (although other gods may also grant it), in which a so-called "red rage" nearly blots out the conscious mind, allowing a warrior to fight with almost supernatural strength and stamina. A warrior in the state of berserkergang may not be capable of distinguishing friend from foe, and may be a danger even to their own allies and loved ones. A warrior who chooses to accept the gift of berserkergang is temporarily trading their rational mind for raw power.

Source: The idea of berserkergang, or "going berserk," is found throughout Norse folklore and legend. Parallels are found in related folklore, like the "warp spasm" of Celtic myth. In Norse folklore, bersekergang is linked to other, even more mystical abilities, such as shapeshifting into animals (e.g. boars, bears, and wolves) or projecting out of the unconscious body a kind of giant animal form (said to be the fylgja, or "fetch") to battle on the berserker's behalf.