Beserkers were Norse warriors--part of the Vikings. They were known for battling in an uncontrollable, trance-like fury. Adding to their ferocity, they would wear bear and wolf pelts when they fought.
This is where they got their name too. 'Ber' is Old Norse for bear while 'sekr" means shirt or coat. This name gave rise to the English word 'beserk,' which accurately describes the Beserkers' behavior.
When the Beserkers went, well, beserk, it is said they could do things that normal humans could not. The fury would start with chills and teeth chattering and give way to a purpling of the face and a great, uncontrollable rage.
While some researchers believe the Beserkers simply worked themselves up into a rage before fighting, others believe they consumed materials that aided this transition.
Some theorize that the Beserkers took hallucinogenic mushrooms or large amounts of alcohol. Others say the fits of rage were the result of self-induced hysteria, epilepsy or even mental illness.