Berserker(Beowulf)

Berserker is the Berserker Class Servant of the Third Holy Grail War of Fate/Corruption. He is a unique Servant because he wasnt' summoned by a normal master, but instead another Servant. In this case it was Avenger, which was actually the deity Angra Mainyu. Angra Mainyu killed the master that was originally going to summon Berserker and took over the mantel itself, meaning that the Einzbern girl of that war summoned Avenger, and Avenger summoned Berserker; the Einzbern girl has often been seen controlling Berserker in Angra Mainyu's stead, but only occasionally. The reason Angra Mainyu needed Berserker was because, while it was inside of Avenger, it couldn't defend itself at all, so it needed a powerful Servant to use as a weapon.

Identity
Beowulf