Zouken Matou, originally Makiri Zolgen, led the Zolgen magus family, exiled from Russia over five centuries prior. He settled in Japan, adopting the Makiri name before shifting to "Matou." Together with Justeaze Lizrich von Einzbern and Nagato Tohsaka, he established the Fuyuki Holy Grail War system, personally designing the Master-Servant framework and Command Seals. His initial goal was idealistic: to eradicate human suffering and reach the "Root" through the Grail's power. He revered Justeaze, who sacrificed herself to become the Grail's core, and vowed to honor her by seeing the Grail completed. Centuries of Grail War failures and his own physical decay corroded these ideals. Zouken extended his life using parasitic Crest Worms, eventually forgetting his original purpose and fixating solely on immortality. Facing the weakening Matou bloodline and dwindling magical heirs, Zouken employed brutal methods to preserve the family's magus standing. He adopted Sakura Tohsaka after her father designated her sister Rin as sole heir. Zouken subjected Sakura to years of torture, implanting Crest Worms to forcibly develop her magic circuits and mold her into a vessel for the Grail's corrupted spirit, Angra Mainyu. This included daily immersion in a pit of devouring insects, poisoning her food to inflict pain, and systematic abuse to break her will. He manipulated his own descendants: deriding Shinji Matou for lacking magic, exploiting Byakuya Matou's passivity, and coercing Byakuya's brother, Kariya Matou, into the Fourth Holy Grail War. Zouken implanted Kariya with flesh-consuming Crest Worms in exchange for power, promising Sakura's freedom if Kariya won the Grail—a promise he never meant to keep. Following the Fourth War's conclusion, he collected Grail fragments and implanted them within Sakura, transforming her into an incomplete Lesser Grail. During the Fifth Holy Grail War, Zouken initially observed, letting Shinji serve as Rider's Master. After Shinji's incompetence caused Rider's defeat, Zouken intervened directly. Using the remains of the defeated Assassin, he summoned "True Assassin," becoming an active Master. He exploited Sakura's link to the Grail's corruption, manifesting as a deadly "Shadow" that consumed Servants and civilians to fuel her transformation into the Grail vessel. Sakura's descent into madness, driven by the Shadow and her turmoil over Shirou Emiya's protection, spurred her rebellion against Zouken. Attempting to possess her body by merging through his Crest Worms, he was forcibly ejected when she crushed his core worm. His final remnant, a solitary worm, sought the Grail during the war's climax. Encountering Illyasviel von Einzbern wearing the "Dress of Heaven"—reminiscent of Justeaze's sacrifice—his long-lost memories resurfaced. Recalling his original love for Justeaze and dream of human salvation, he abandoned his quest for immortality and accepted death peacefully. In non-Heaven's Feel timelines (Fate and Unlimited Blade Works routes), Zouken's involvement is minimal. Speculation suggests he may have been killed by Kirei Kotomine or Gilgamesh to prevent interference in the Grail War, though this remains unconfirmed.