Hassan-i-Sabbah (ハサン・サッバーハ, Hasan Sabbāha?), Class Name True Assassin (真アサシン, Shin Asashin?), is the "True" Assassin-class Servant of Faldeus Dioland in the True and False Holy Grail Wars of Fate/strange Fake.
Assassin remains always hidden, floating in darkness as a white skull mask that every Hassan wears, although he can also appear as a humanoid shadow. When revealing himself to his master, he only does so surrounded by darkness and vanishing as soon as light appears. Assassin's voice is coldly and inhuman, but Faldeus was able to tell that it is a male's voice despite being unable to imagine Assassin's age or build from it.
Upon meeting his Master, True Assassin questions Faldeus' convictions and whether or not they are worth taking the lives of others for, declaring that is what it is to form a contract with him. He agrees to be his Master's Shadow so long as he does not abandon his convictions. Faldeus believes that if he got a single answer wrong, Assassins would probably have killed him. Assassin speaks in an old manner, for example using "thee" instead of "you".
When Faldeus first summons True Assassin, the Servant doesn't appear, causing Faldeus to believe that the summoning has failed. However, it's revealed that True Assassin was able to hide himself, even from his Master, communicating to Faldeus by writings. True Assassin speaks with Faldeus, though he remains hidden from him, asking if Faldeus has strong faith in something and if he is ready to kill. As Faldeus answers positively to both, Assassin agrees to make a contract with him.
As more and more people are infected by an unknown disease in the city of Snowfield, where the Holy Grail War is being held, Assassin is shown being aware of the one causing this epidemic, calling it a curse-plagued wind, and informing Faldeus that it shouldn't be able to get inside their base of operations. When Faldeus becomes more concerned about Bazdilot Cordelion, the Master of True Archer and a member of the Scladio crime family, he decides to send Assassin to kill the head of the family, Galvarosso Scladio, who is currently residing at the other side of the country. Before departing on his mission, Assassin asks one last time if Faldeus has the faith necessary to kill, warning that if it is proven later that he does not "the curse will rebound and devour him".
Assassin infiltrated the main Scladio residence, and observed a conversation Galvarosso had with his great-great-granddaughter Olivia, where he gave her a plush fox. Galvarosso removed his respirator and asks Assassin if the darkness that signals his end is here, stating that he knew for years that the moment of his death would come after Olivia asks him for a plush fox, as that was the only vision his Mystic Eyes of Premonition would show him, yet he did not deviate from the vision and try to change his fate as he would rather accept death than watch the youngest member of his family cry and be sad. Galvarosso begs Assassin to also end the plot of the magi in his organization; they gave up on trying to prolong his life and decided that his personhood doesn't matter, to Bazildot's objection, and have killed the souls of powerful Americans and overwrote their personalities to match his. Assassin covers Galvarosso's eyes, revealing his presence, and telling him not to fear. Assassin takes the form of a humanoid shadow, and before ending Galvarosso's life, he tells him: “I have no reason to judge thee, nor do thou. Entrust all to the night. Awake in peace on the far side of sleep.”
Faldeus recieves a call from a general informing him that on the previous day alone, a promising presidential candidate was among thirty-five influential people in the worlds of finance, media, and politics to die suddenly from accidents or natural causes. After comparing the times and places of death, Faldeus was able to find a pattern based on the proximity to Galvarosso Scladio’s headquarters. Faldeus is unable to confirm whether Galvarosso is alive or dead, but figured that Scladio mages would conceal the news of his death for some time.
Faldeus questioned where Hassan is and what he is doing, as he realizes that the effects of the Holy Grail ritual is spreading to the rest of the United States, and is no longer confined to Snowfield to the pleasure of Francesca. After False Rider causes several participants of the Holy War, including some of Faldeus' soldiers, to disappear into Kuruoka's dream world, Faldeus prepares to contact Assassin but receives a signal from him instead, and goes to his workshop to meet with him. Faldeus questioned Assassin on his actions, but Assassin confirms that he have not deviated from his agreement and that he ended the lives of the people who were Galvarosso Scladio, who had his personhood eaten away.
Assassin's Presence Concealment is EX, and drops to A+ when he starts to attack. He is able to become one with the darkness and gain surrounding magical energy from it, as long as he does not materialize he almost doesn't need his Master's magical energy supply. He can obscure his other parameters and Noble Phantasm even from his own master as long as Command Spells are not used. Assassin is very skilled in acting in secrecy, being able to write down words on paper and slip them into his hand, even going so far as to manipulate his Master at first into writing them, without even Faldeus himself noticing. Assassin's voice seemingly comes from every direction, and yet it only audible to a single person, and it caused Faldeus to prepared for his own death and become convinced that it is certain and impossible to avoid, despite the voice lacking bloodlust. Assassin's presence left Faldeus immobilized and sensing nothing, as if he was swallowed in limitless blackness, even almost wondering if his ears are playing tricks on him as he hears Assassin in the airless void.
Assassin is capable of entering the most secure of areas undetected, as he was able to infiltrate the heart of the main Scladio Residence, a composite workshop made by elite magi with a variety of defense mechanisms and numerous evil spirits inside a powerful, thirty-five-layer ward, which was surpassed only by the protection of the Clock Tower, the Wandering Sea, and labrynth magi powerful end to approach the root would spend their lives on. Assassin is also capable of inducing death through seemingly natural accidents and illnesses, with the majority of illness he prefers to use being sudden strokes and heart attacks.
Assassin can manipulate technology to some extent, as he caused rows of monitors to flicker with noise, inlcuding monitors powered by magecraft and connected to familiars, and then revealed himself to Faldeus by making him pay attention to a single monitor that started to emit noise, despite Assassin supposedly being outside the prison Faldeus is using as a base. He was later able to shut off and turn on fluorescent lights in a corridor at will.
 Fate/strange Fake - Volume 2 - Servant status: True Assassin
Class: True Assassin
Master: Faldeus Dioland
True name: Hassan-i-Sabbah
Strength: Even Faldeus cannot confirm
Endurance: Even Faldeus cannot confirm
Agility: Even Faldeus cannot confirm
Magic: Even Faldeus cannot confirm
Luck: Even Faldeus cannot confirm
Noble Phantasm: Even Faldeus cannot confirm
■ Presence Concealment: EX
Becomes one with the World itself. The moment one shifts to attack, it becomes A+.
■ Shadow Lantern: A
Skill that allows one to become one with the shadows. Because he can gain surrounding magical energy from the darkness, as long as he does not materialize he almost doesn't need the Master's magical energy supply. As long as a command spell is not used, it is possible to conceal one's status from the Master.
■ (Obscured in volume 2)
Maximum Targets: ???
- Fate/strange Fake Volume 2, Chapter 5: "A Shadow in the Park"
- Fate/strange Fake Volume 3, Chapter 7: "The Canon of the Demigods"
- Fate/strange Fake Volume 3, Chapter 9: "A Horse Not Yet Pale; Mud Yet Firm"
- Fate/strange Fake Volume 5, Chapter 16: "Breaking Dawn and Wakeless Dreams I"
- Fate/strange Fake Volume 4, Bridge Chapter: "One Day, Above the Sky"