Avenger is the Avenger-class Servant of the Einzbern family in the Third Holy Grail War. He is an abnormal Servant that is not part of the regular seven classes, whose true identity is Angra Mainyu. While he was not actually the evil god of Zoroastrianism, his defeat in the Third War corrupted the Grail, turning it into the equivalent of All the World's Evils.
This manifestation attempts to be born through the Holy Grail, and acts as the Servant of Sakura Matou in the Fifth Holy Grail War of the Heaven's Feel route of Fate/stay night.
He acts as the Servant of Bazett Fraga McRemitz in place of Lancer in the four day loop of Fate/hollow ataraxia.