The idea of a warrior who becomes savage and fierce and uncontrollable makes sense in the narrative kind of war that revolves around lots of individual fights between warriors, but seems totally out of place in battles like the ones I understand were actually fought in the 9th-11th centuries (shield walls pressed against each other). Did Berserkers exist at all other than as a literary invention? Were they shock troops who flung themselves at a wall? Were they only "savage and uncontrolled" after an opposing force broke?