Heroic Spirit of berserker rage. A type of Servant that specializes in “going insane” and destroying things, regardless of their basic parameters.
The Berserker in the story was already an excellent Heroic Spirit, but due to the “maddening” he was transformed into an unstoppable monster.
If he was contracted to a normal Master, he would be capable of canceling their command spells, but as the Holy Grail Illyasviel has special command spells that can even reign in the unruly Berserker.
His true identity is the great hero of Greece, Heracles.
Given that he went mad on numerous occasions in his myths, being summoned into the Berserker class must have felt like meeting an old friend.
Of course, Heracles, who is stout of heart, body, and mind on top of being skilled with all sorts of weapons, is more than qualified for the Saber class as well.
If he had been summoned as a Saber, he would have been able to demonstrate his splendid sword techniques to his heart’s content.
He was summoned one… no, two months before the Holy Grail War began.
Because his emotions were sealed he was incapable of acting on his own.
But, no matter how strong the binding of the command spells was, the sliver of reason at his core remained intact.
The reason he was able to stop his axe sword at the last second even after being blackened in the Sakura route, was probably due to the strength of his heart that had been afflicted with madness so many times in life.
He is, without a doubt, the manliest character in all of Fate/stay night.

Fate/side material: Fate Encyclopedia