The Servant of Kotomine Kirei in the 4th Holy Grail War.
His unique trait is the capability to manifest as multiple entities at once.
The multiple bodies of Assassin can vary widely in age, gender, build, and style of dress, but in general, their hair appears purple, and their skin and clothing are nearly black. They wear the standard skull mask associated with the known Hassan of the other Grail Wars.
Not to be mistaken with Sasaki kojirou.
True Name: Hassan-i-Sabbah