Assassin's true identity is Hassan-i-Sabah, the legendary Persian leader of the Assassins and the infamous "Old Man of the Mountain."
In the 4th Holy Grail War, Assassin is summoned as the Servant of Kirei Kotomine. What sets him apart from other Servants is his ability to split into multiple independent bodies simultaneously, making him uniquely dangerous as an assassin—he can be in several places at once, which is pretty much the ultimate advantage for sneaking around and taking targets out.
Content compiled by AnimeList.moe from publicly available sources.
