Hassan-i-Sabbah (ハサン・サッバーハ, Hasan Sabbāha?), also known as the "Old Man of the Mountain" (山の翁, Yama no Okina?), is the pseudonym shared by the nineteen leaders of the Hashshashin, a medieval Islamic sect based in Persia. All nineteen are currently wraiths considered Heroic Spirit Candidates that have yet to properly ascend to the Throne of Heroes. When summoned as Servants, they are always the Assassin class, and sometimes the only possible candidates for the class entirely. The name of the sect is the etymological root of the word "assassin;" and the class name itself thus acts as the catalyst to summon the members of the organization.
The requirement for those who might truly be described as perfect assassins would normally be to remain anonymous and consequently fail to become Heroic Spirits, so only the Old Man of the Mountain who is known throughout history would normally be applicable for the Assassin class. They are normally summoned in place of truly anonymous killers, but others have managed to fit into the class as well. They have all lost their faces and names upon becoming wraiths, instead only having carved out faces hidden behind skull masks. If they were to regain them, they may be able to be properly inducted. Due to being selected from a group of Wraiths, their abilities are low. The greatest skill of each is expressed as their Noble Phantasm, which all share the name Zabaniya—the name of the 19 angels that guard Hell and torment its inhabitants under Archangel Maalik, according to Islamic faith.
|1||"Old Man of the Mountain"||Unknown||None|
|?||Hassan of the Cursed Arm||1200s||Delusional Heartbeat|
|?||Hassan-i-Sabbah (Third Holy Grail War)||Unknown||Cyber Phantasy|
|?||Hassan-i-Sabbah (Fate/kaleid liner PRISMA☆ILLYA)||Unknown||Dead Heartbeat Melody|
|?||Hassan of Intoxicated Smoke||Unknown||Unknown|
|?||Hassan of Serenity||Unknown||Delusional Poison Body|
|?||Hassan-i-Sabbah (Fate/strange Fake)||Unknown||Unknown|
|?||Hassan of the Quaking Pipe||Unknown||Unknown|
|?||Hassan of Shadow-Peeling||Unknown||Unknown|
|?||Hassan of the Shining Star||Unknown||Unknown|
|?||Unknown||Unknown||Ichor of Reverie|
|?||Unknown||Unknown||Raving Shadow Flash|
|?||Unknown||Unknown||Unfeeling Patrolling Spirits|
|N/A||No Name Assassin (#19 candidate)||Unknown||Phantasmal Pedigree|
|19||Hassan of the Hundred Faces||Unknown||Delusional Illusion|
 Fate/side material - Encyclopedia: True Assassin [Servant], p.062 [T]
True Assassin [Servant]
The real Heroic Spirit of assassination.
He was summoned by Matou Zouken, materialized using the flesh of Sasaki Kojirou.
Bearing a white skull mask and black cloak, he was the classical assassin.
An unremarkable Servant in terms of abilities, but highly utilitarian. In contrast with his disturbing appearance, he was very loyal. He accepted the authority of his master and would never betray him.
Strictly speaking, True Assassin was merely a member of a group of Wraiths (Heroic Spirit candidates) who bore the name "Assassin", thus his abilities were intrinsically low. In order to compensate for this disadvantage, he modified his body. As a result, he was able to rival other Servants.
There was no face under the mask. The skin and nose were shaven off. He inherited the seat of the Old Man of the Mountain as a "nobody".
He lacked intelligence after materialization, but that was remedied dramatically after acquiring Lancer's heart. Since then, he was also influenced by Lancer's personality.
By the way, True Assassin was particularly attached to his Dirks. He would carefully pick all of them up after each battle.