Berserker

"One text describes them as "mad as dogs or wolves" and "strong as bears or wild bulls". They had the strange habit of chewing the edge of their shields before battle and emitting roars and growls."

- R.G. Grant, "Warrior: A Visual History of the Fighting Man", pages 52-53

The regular Vikings warriors of old where formidable hit-and-run raiders, yet their prowess in battle and savagery was nothing compared to those whom all feared, those whom the Vikings used effectively as bestial shock troops, those who had the strength of wild beasts: the Berserkers. The Berserkers where bear-skin clad warriors who fought with no armor and in a crazed frenzy brought about by their rituals and chants. Some wore these skins over most of their body, with the head and jaw skins over their heads and faces, while others wore the skins like a cape, with the upper head and ear skins covering the tops of their heads and exposing their faces. The latter went into battle otherwise naked. A similar group fo warriors, the wolf-skin clad Ulfhednar (pronounced Ulf-heth-nar), where just as powerful and mad as the Beserkers, and eventually where absorbed by them.

When Berserkers fought they where said to be immune to arrows, swords and fire, to be s strong as bears or bulls, and immune to pain. It seems that, whatever the rituals and chants that brought about their inner, bestial natures, also brought about major adrenaline surges, increasing their strength and speed to superhuman levels (upwards of 10 times their normal strength: beyond this and ligaments and joints will tear), rendering them immune to pain, and lessening their blood flow. Many of the ideas we have of Vikings stem from the Berserkers. The Ulfehdnar, in particular, due to their powers in battle and dress, was a major influence of European Werewolf Legends.

The "Wendol" of Michael Crighton's "The 13th Warrior/Eaters of the Dead", which where neanderthals in bearskins, where inspired by Berserkers.