Character: Assassin

Assassin is actually the reincarnation of the head of the Assassin Order. While every Assassin in every Grail War thus recorded has shared the same name of Hassan-i Sabbāh, they are different persons as all the leaders of the Assassin Order changed their names to Hassan-i Sabbāh upon ascending to the position. The Hassan-i Sabbāh in the Fourth War became head by intentionally fracturing his psyche during life to confuse opponents, a form of self-inflicted dissociative identity disorder. When summoned as a Servant, this ability manifests itself as the Noble Phantasm Zabaniya: Delusional Illusion, which splits his body into up to eighty different entities, sharing his spiritual potential amongst them. They act as reconnaissance agents for Tokiomi and Kirei after Gilgamesh "kills" one of them. Assassin can only be truly defeated if all of them are killed.

Source: Wikipedia