Berserker (バーサーカー, Bāsākā?) is the Berserker-class Servant of Illyasviel von Einzbern in the Fifth Holy Grail War of Fate/stay night.
Berserker is Heracles (ヘラクレス, ?), also known as Hercules, the greatest hero in Greek mythology. He was a demi-god born from Zeus, the king of the gods, and a human woman. While he had superior talent in all areas, he had a number of personality issues. He beat his harp teacher to death over a small matter during his childhood, so he lived as a shepherd afterward to calm his temper. He thus became a splendid man who accomplished several great deeds that caused the King of Thebes to grant Heracles his daughter, Megara. They had two children, and he earned the right to succeed the king.
He was both young and strong and it seemed that there was no match for him among humans. His future seemed bright, having married the king's daughter and bearing two children, but Zeus's wife, Hera, detested the child born between Zeus and a human. She meddled with his fate at every opportunity, and she started by sending madness to him that caused him to kill his family. Troubled by the sins he had committed, he received a sign that he should atone by becoming the slave of Eurystheus, the man who took away his promised position as king.
Eurystheus, jealous of the strong hero, gave him many difficult tasks, and claimed that Heracles would remain his slave until they were completed. These difficult labors were tasks impossible to complete with normal human skills that later came to be known as the famous Twelve Labors of Heracles. The number of labors originally chosen by the gods was ten, but Eurystheus added two more labors after denying the fulfillment of two of them. Heracles managed to complete even those tasks, so he was granted not only his freedom, but he also received an immortal body as proof of his great accomplishments.
This marked the beginning of the great hero Heracles, the greatest hero in Greece, who singlehandedly achieved great deeds equaling those of the Trojan War and the Argo Expedition. Despite that, Hera's hatred was endless, so the rest of his life was again full of madness. While he should have been immortal, he was killed by poison by one of his wives. Though he was a great hero equally matched with the sun god even as a human, he was slowly killed by Hera's persistent jealousy. He burned his own body, still contaminated with poison at the time of his death, and he entrusted himself to Zeus' judgment. The gods approved of his great deeds in a conference, prepared a seat in Olympus after his death, and acknowledged him as a god.
Berserker is a tall, statue-like man who is described as a "black giant." He gives off the impression of a monster, instantly marked as the Berserker-class Servant by Rin Tohsaka, in both appearance and presence. He imparts fear unto Shirou Emiya, freezing him in place.