Zouken Matou

"There were terrible cries for the first three days, but by the fourth day, she was silent. Today, she was thrown at dawn in the worm storage to test how well she would last, but, ho ho, she endured it for half a day and is still breathing. What do you know, the Tōsaka material isn't defective."

- Zouken Matou, Fate/Zero

Zouken Matou is the grandfather of Shinji and Sakura Matou and a villain from the Fate series. He first appears in the Heaven's Feel route. Zouken is actually around two centuries old, having used parasitic worms to continually expand his lifespan. In Heaven's Feel, he summons True Assassin from the corpse of Assassin, thus becoming the seventh Master in the fifth Holy Grail War.

Background
Zouken was originally known as Makiri Zolgen and is the patriarch of the Matou family. The family later changed later became known as Makiri when he changed his name to Zouken Makiri, and it changed again upon making it into the "Matou" family.

Alongside Nagato Tohsaka and Justeaze Lizrich von Einzbern, he was one of the creators of Fuyuki's Holy Grail system along with Kischur Zelretch Schweinorg guidance. Back then, the true purpose of the Holy Grail was to reach the Gate of All Things, the root of the universe. Zouken proposed the system of "Master" and "Servant" while Nagato provided his property Fuyuki and Justica developed the Heaven's Feel system, offering the homunculi vessels for the Grail and becoming the core of the Great Grail. Justica would sacrifice herself by transforming into the Grail's magic circuit.

At the time, Zouken admired Justica so deeply that he was devastated to see her sacrifice. He decided to ensure that her sacrifice would not be in vain by keeping himself alive long enough to see the moment the Grail is completed and reaches the heavens. However, as time went on, his frustration caused by the incomplete Grail and the deterioration of his body over such a long period of time slowly caused him to forget his original intention and left him only with abnormal obsession with immortality.

Shrewd, powerful, and ancient, he is known today as Shinji and Sakura's grandfather and the uncle of Kariya and his brother Byakuya. He conceals his status as the Matous' ancestor by masquerading as a relative to his descendants, which some of his family members have long suspected given how repeatedly Zouken's name has appeared in the family tree. He expresses a large and extensive library of sorcerous knowledge, but his lessons are often strict and harsh. He believes Shinji to be a disgrace as a sorcerer and believes Sakura to have more potential than her brother.

Since the Matou blood has thinned and no longer has heirs naturally capable of sorcery, Shinji was incapable of being a true sorcerer so Zouken offered to adopt Sakura Tohsaka into the Matou family when her father, Tokiomi Tohsaka, decided to only tain his older daughter, Rin Tohsaka, to become his successor.

Sakura was implanted with Zouken's magical worms in preparation for her usage as a vessel for Angra Mainyu during the fifth Holy Grail War.

Abilities
Zouken is a powerful magus with extensive knowledge. He cannot be killed by normal or physical means because his body is made out of the worms he uses to prolong his life span. Fatal wounds can easily be shrugged off, and he can easily reform his body by devouring a human even if his body is destroyed.