Summoned by Zouken Matou during the Heaven's Feel route using Caster's deceased Assassin-class Servant as a catalyst, True Assassin is a Servant of the Holy Grail War. His true identity is Hassan-i-Sabah, the "Old Man of the Mountain," legendary founder and leader of the Middle Eastern assassination order al-Assasīn. This organization employed political killings and consumed hashish to attain superhuman mental states for missions, with the term "assassin" originating from the Arabic word for hashish users. Hassan-i-Sabah is a collective title; each leader sacrifices their original identity and appearance upon assuming the role. This summoned Hassan is one such leader, stripped of his true name and individuality. His appearance emphasizes concealment and abnormality: a slender figure draped in tattered ebony robes, wearing an ashen skull mask. His extensively bandaged right arm resembles a useless flat board incapable of normal function. Removing the mask reveals a skull-like face devoid of skin, reflecting the identity loss inherent to becoming Hassan-i-Sabah. As an Assassin-class Servant, his capabilities focus on stealth and indirect combat. He possesses the class skill "Presence Concealment," enabling him to observe even high-level Servants like Saber undetected. His preferred weapons are throwing knives called Dirks, hurled precisely at vital points. He exhibits natural protection against wind-based attacks, likely developed from enduring harsh desert sandstorms. His Noble Phantasm is "Zabaniya: Delusional Heartbeat." This manifests as his bandaged right arm transforming into a long, spectral limb capable of remotely extracting and crushing a target's heart, replacing it with a magical duplicate. This technique bypasses physical armor but can be countered by sufficient magic resistance. Summoned early in Heaven's Feel, True Assassin operates under Zouken's command. His first significant act is ambushing and killing Lancer using Zabaniya, then devouring Lancer's heart to gain normal speech. He later assists Zouken in ambushing Shirou Emiya and Saber at Ryudou Temple, acting as a distraction so the shadow can consume Saber. Rider thwarts his attempt to kill Shirou. He participates in the assault on Einzbern Castle with Zouken and Saber Alter, contributing to Berserker's defeat and consumption. On Zouken's orders, he infiltrates the Emiya residence undetected to deliver an invitation to Shirou. During an attack on Kirei Kotomine to capture Illyasviel, True Assassin attempts Zabaniya on Kirei but finds it ineffective. Kirei binds him and deals a fatal blow to Zouken. Sakura Matou, after betraying and killing Zouken, removes Assassin's mask. Seeing his featureless skull face and remarking on his lost identity, Sakura devours him using the shadow. He demonstrates consistent loyalty to Zouken Matou and a preference for stealth and carefully planned ambushes over direct confrontation.