True Assassin is Hassan-i-Sabbah, specifically "Hassan of the Cursed Arm," a historical leader of the radical Middle Eastern assassination sect al-Assasīn. This organization achieved its goals through political assassinations and psychoactive substances, with the name "Assassin" originating from the Arabic "hashish." Hassan operated from a mountain fortress, earning the epithet "Old Man of the Mountain" and establishing a legacy maintained by successors who adopted his identity. He was summoned abnormally during the Heaven's Feel route. Disgusted by Shinji Matou's incompetence and Rider's defeat, Zouken Matou exploited the corpse of Caster's irregularly summoned Assassin servant as a catalyst to summon the genuine Hassan-i-Sabbah. This act bypassed standard Holy Grail War rules, designating him "True Assassin" to distinguish him from the previous imposter. Visually, True Assassin manifests as a slender figure shrouded in tattered, dark garments. His face remains perpetually hidden behind a skull mask, later revealed as his actual fleshless visage. His heavily bandaged right arm appears superficially useless but conceals his primary weapon and Noble Phantasm. His capabilities derive from the Assassin class, granting exceptional Presence Concealment that allows him to observe undetected even by powerful servants like Saber. His combat style relies on precision strikes with small, black throwing daggers (Dirks) targeting vital points. He possesses innate protection against wind-based attacks, developed during his lifetime in harsh deserts. To gain normal speech, he devoured the spirit core of the defeated servant Lancer. Within the Heaven's Feel III. spring song narrative, True Assassin serves exclusively as Zouken Matou's contracted servant. He acts solely on Zouken's commands, functioning as an instrument in schemes to manipulate the Holy Grail War and achieve immortality. Key actions include assisting Zouken in combat against Kirei Kotomine during the raid on Einzbern castle and attempting to eliminate threats to Zouken's plans. His story concludes during Sakura Matou's confrontation with Zouken. After Zouken reveals his true form as a crest worm embedded near Sakura's heart and attempts to possess her, Sakura retaliates violently. She physically tears Zouken's worm form from her body and crushes him. Immediately following this, under the corrupted Grail entity Angra Mainyu's influence, Sakura uses the Shadow to absorb True Assassin, dissolving his spiritual form and ending his manifestation.