True Assassin served as Zouken's Servant in the fifth Holy Grail War. He can only be seen on the Heaven's Feel route. True Assassin is loyal to Zouken and prefers to avoid direct combat unless he has a plan to win the battle.
As an Assassin class, True Assassin is not made for melee combat. Instead, he is exceptionally skilled at stealth and espionage. He is also resistant to wind-based attacks due to him enduring sandstorms during his lifetime and he uses throwing daggers.
Hassan-i Sabbah is the leader of a legendary assassination organization in the Middle-East. Also known as "The Old Man of the Mountain".
Hassan-i Sabbah is part of a large religious organization in the Middle-East, and they practice a very strict and fanatical doctrine. Hassan was a part of a smaller group which practices the most brutal methods. They used assassinations as a political tool. The assassins of the group use a drug called hashish to attain an enhanced mental state to reach superhuman capability. The word "assassin" is said to be derived from the word hashashin, meaning "user of hashish" in Arabic.
Hassan-i Sabbah was responsible for boosting the organization's reputation as the most feared group in the world. He occupied a castle on a mountain and used it as his base of operations. The assassins under Hassan are unrivaled in their skill in assassination.
Hassan-i Sabbah is said to be a title given to the most skillful assassin of the organization, who will also be their leader.
True Assassin wasn't seen on the Fate and Unlimited Blade Works routes.
Zouken summoned True Assassin early in the route. He took over Assassin's shell after he was attacked by the black shadow. The next day, Lancer investigated Ryudou Temple. True Assassin killed him with Zabaniya and the black shadow devoured him. After Zouken was injured by Archer, they met in the forest and formed a contract.
The following day, Zouken and True Assassin ambushed Shirou and Saber at Ryudou Temple. True Assassin distracted Saber to allow the black shadow to devour her. He then attempted to kill Shirou but was saved by Rider.
Much later, Zouken, True Assassin, and Saber assaulted Einzbern castle. They defeated Berserker and the black shadow devoured him. 3 days later, Zouken sent True Assassin to invite Shirou to the Matou house. He was able to sneak in undetected.
2 days later, Zouken and True Assassin attacked Kotomine to retrieve Illyasviel. True Assassin tried to use his Zabaniya on Kotomine but he discovered that it doesn't work against Kotomine. He then bound True Assassin and dealt a fatal blow to Zouken.
On the last day of the war, Sakura betrayed Zouken and killed him. She took True Assassin's mask off and discovered that he has no face. She then devoured him.
True Assassin's throwing knives.
True Assassin carries an unknown number of them, and he is able to quickly and precisely throw several of them at once, targeting different points of the body simultaneously.
Zabaniya, Heart of Delusion
The arm of the evil spirit Shaytaan. It is a long and skinny arm that is usually wrapped in a long bandage.
True Assassin uses it to swap his target's heart with a clump of magic energy. He can then destroy the target's heart, killing his target. Zabaniya ignores armor. The target must have a high enough magic resistance to prevent the implant of the magic energy.
True Assassin used it against Lancer, Saber, and Kotomine on the Heaven's Feel route.