Originally Makiri Zolgen, Zouken Matou belonged to a magus family originating in Kyiv roughly five centuries before the depicted events. Driven from their homeland, his family eventually settled in Japan, altering their name first to Makiri and later to Matou. As one of the three founders of the Fuyuki Holy Grail War system alongside Nagato Tohsaka and Justeaze Lizrich von Einzbern, he proposed the Master-Servant framework and designed the Command Seals used to control Servants. His initial motivation sprang from an idealistic desire to eradicate human suffering and reach the "Root" of all knowledge through the Holy Grail. He deeply admired Justeaze, who sacrificed herself to become the Grail system's core. Devastated by her loss, he vowed to prolong his life until witnessing the Grail's completion to honor her sacrifice. Centuries of existence caused his body to deteriorate, forcing him to sustain himself using Crest Worms—magical parasites he developed—transferring his consciousness between hosts. This prolonged life gradually eroded his original ideals, replacing them with an obsessive pursuit of immortality. He manipulated successive generations of the Matou family, presenting himself as a relative while concealing his true identity as the patriarch. By the time of the Fourth and Fifth Holy Grail Wars, his motives narrowed solely to obtaining eternal life through the Grail. The Matou magical lineage waned over generations, leaving descendants like his grandson Shinji Matou incapable of sorcery. To secure an heir, Zouken arranged Sakura Tohsaka's adoption from the Tohsaka family one year before the Fourth Holy Grail War. He forced brutal training upon her, implanting Crest Worms into her body, causing immense physical and psychological agony. These worms fed on her magical energy, intended to transform her into a vessel for Angra Mainyu, the corrupted spirit within the Grail. He further poisoned her food to increase her pain tolerance and break her spirit, ensuring her suitability. After the Fourth War, he embedded fragments of the destroyed Grail within Sakura using Crest Worms, turning her into an incomplete Lesser Grail. During the Fifth Holy Grail War, Zouken initially remained passive, allowing Shinji to act as Sakura's proxy Master of Rider. Following Rider's defeat and Shinji's failures, he intervened directly, summoning "True Assassin" from the remains of the original Assassin Servant. He exploited Sakura's connection to Angra Mainyu—manifested as a destructive shadow—to eliminate other Masters and Servants. Sakura's eventual corruption by the shadow exceeded his control, leading her to absorb Servants independently and reject his authority. Kirei Kotomine destroyed his physical form during an assault on the Einzbern castle, though he survived through worms hidden within Sakura. In his final moments, as he attempted to reach the activated Grail, Illyasviel von Einzbern—appearing reminiscent of Justeaze—triggered his buried memories of his original ideals. Confronted with his failure to honor Justeaze's sacrifice or achieve humanity's salvation, he accepted death peacefully. In other Fate/stay night routes like Fate and Unlimited Blade Works, Zouken lacks direct involvement. Scenes in the Unlimited Blade Works anime adaptation suggest Kirei Kotomine may have eliminated him indirectly, potentially orchestrating his demise through Gilgamesh.