Berserker is a fictional character from the Japanese visual novel and anime series Fate/stay night by TYPE-MOON.

The Servant of Illya, he is a swarthy giant with adamantine skin and immense strength. He wields a colossal stone Axe-Club, and is capable of causing massive destruction with the mere backlash of his swings. As a Berserker, he was stripped of his sanity and reason, acting feral and animalistic in combat. Berserkers are praised as members of the "Strongest" class, though they are difficult to control. Illya, however, appears to have little trouble managing Berserker.

The voice acting for Berserker is done by Tadahisa Saizen in Japanese and Michael McConnohie in English.

Source: Wikipedia