A monster that used to be human, who is now reduced to nothing but an angry beast without reason or understanding of the world around them. Uses their herculean physical strength and endurance to destroy everything that appears before them, a requirement is that the hero who is summoned as Berserker must once in their life have gone berserk. Alternatively it could also be a curse laid upon any hero, though it is not sure if such a curse would benefit them; say summoning a caster and putting the curse on them.
Example Heroes: Hercules, Lancelot
To force the servant you are summoning into the class of Berserker you just add these three lines to the end of your summoning chant.
"Yet, thou serves with thine eyes clouded in chaos.
Thou, bound in the cage of madness.
I am he who commands those chains."
As listed under the Servant Category Page
Their free Class skills are:
Madness Enhancer Rank D
Berserkers are then allowed to pick up to six skills for up to 37 points.