Avenger♂

Avenger was an ordinary villager. One day, he was randomly chosen as the "source of all evil in the world" by his village and was tortured and sacrificed in the ensuing rituals. The villagers, who were suffering from various disasters, eventually came to need a scapegoat for blaming their troubles onto. So they continuously tortured him until he went insane. They captured him, beat him, carved every word that cursed mankind onto his body, forced every sin imaginable upon him, took out bits and pieces of him slowly, defiled his mind with absolute evil, and held him responsible for all of it in the world.
He was summoned in the Holy Third Grail War by the Einzbern family. The only Avenger Class Servant summoned was during the Third Holy Grail War, and rather than a true Heroic Spirit, an Anti-Hero was summoned. He was quickly killed in four days and was trapped in the wish-granting spiritual Holy Grail. His influence tainted the Grail system. Originally the Grail system can summon only "good" Heroic Spirits, however due to his corruption caused the fourth and fifth Wars to summon irregular spirits.
(spoiler hidden)