The "Berserker" was a warrior employing a type of endo-magic. Berserkers featured heavily during the Mage Wars. Equipped with weapons and armor, they cast a spell on themselves to receive unnatural strength, speed, and size for a short time.
Contemporary accounts tell of warriors growing over twelve feet high, and gaining the strength of ten men. Usually armed with powerful enchanted weapons and armor, a single berserker was an army unto himself. They moved through the battlefield with uncalculating wrath, wreaking havoc on friend and foe alike.
The spell caused a sort of violent mind-fog to overtake the caster, causing him to lose all control over his actions. Berserkers would often turn on their own troops in a blind rage. The typical strategy was for a berserker to break away from the others, fight as deeply into the enemy forces as he could, and cast the spell, where the bulk of his anger would be directed at the enemy.
A casting lasted anywhere from two to ten minutes, though some accounts tell of berserk rages lasting for hours (it is possible this is the result of casting the spell several times). At least one account deals with a "permanently-berserk" fighter-mage whose path of chaos lasted for weeks and for whom an entire army was assembled. Ten thousand men would give their lives to end this one rampage.
Many notable accounts survive of children sired by mages under the spell. They were very rare, but by no means unheard of. Typically such offspring were tall (eight to nine feet) and extremely strong. But they never lived more than fifty to sixty years and were typically considered unintelligent.
The most well known example was a man named Sol, who fought beside Eieber as one of The Ten. Sol was eight feet tall and carried two greatswords (claymores) as his weapons, with one in each hand. His exact age was unknown, but he was probably around 57 when he died of natural causes.
Because of his fame and notoriety, Sol was studied extensively during his lifetime. He had been raised a slave and did not learn to read until he was well into his thirties. He was still a very good strategist and effective leader. Researchers studying him concluded that he had at least average intelligence but was slightly deficient in some areas.
The spell is referenced in many sources as "The Berserk Fury" and was probably quite easy to learn (most berserkers were more fighter than mage). The spell itself may have been created by Nathan Searlin, though no contemporary source directly attributed it to him. Further, no primary source confirms the use of the spell during the First Chaotic Period.
In the Dynastic Period, the spell was already in use. The Marcons in particular trained berserkers by the tens of thousands. They were considered expendable, and according to surviving Marconian military manuals: "if a berserker survive a singe fight and to his name attribute a thousand souls, then his worth be proved"; essentially, if it lasted for a single fight and killed a thousand enemies, the Marcons had their monies' worth.
A single known transcription survived the Mage Wars, and is kept in archives at Stormwind Keep in Arindell, in the private library of the Pendragon. Unconfirmed rumors claim that the transcription was actually burned by Eieber himself, as a deathbed-request from Sol.