Ryūki

is a worshiper of Jashin and the main antagonist of the Konoha Shinden novel and its anime-only arc.

Background
Ryūki dislikes the peace that spread throughout the world after the end of the Fourth Shinobi World War, as he feels it makes the countries and its people weak. To rectify this, he decided to follow in Hidan's footsteps by becoming immortal; he would bring death and destruction back to the world and establish himself as its ruler. To that end, he needed to acquire a large number of sacrifices for him to achieve immortality; preferably by luring and abducting young girls traveling through the mountains of the Land of Hot Water under the cover of a Resurrection hot spring.

Konoha Shinden: Steam Ninja Scrolls
Ryūki meets Tatsumi, a young woman who dearly wishes to talk to her recently deceased mother. Because he has almost acquired the number of sacrifices he'll need, he asks Tatsumi to bring him one more follower in exchange for reuniting her with her mother's spirit. Tatsumi complies and brings her Mirai Sarutobi. Eager to begin making sacrifices to Jashin, Ryūki offers some drugged water to Mirai to drink. She suspects his intentions and refuses. When the other Jashinists attempt to seize her, Mirai reveals herself as a ninja of Konohagakure.

Mirai accuses Ryūki of being behind the recent kidnappings in the area, which he doesn't deny. Tatsumi is horrified to hear this, but he assures her that he will follow through on his promise to reunite her with her mother; he then moves to kill her so that they can meet in the afterlife. Mirai saves Tatsumi but is seemingly injured in the process. Ryūki doesn't think Mirai will offer much challenge but is proven wrong when she starts fighting and defeating the other Jashinists. He tries to make her stop by having the Jashinists take the other sacrifices as hostages, but they are stopped by Might Guy and Kakashi Hatake.

Although he doesn't want to because it will delay his plans for immortality, Ryūki calls upon Jashin via Curse Technique: Body Controlling Manipulated Blood: he ingests some of the blood Mirai spilt earlier, linking her with a puppet he has in his possession. When he stabs the puppet in the heart, Mirai is seemingly killed. And then she gets up, revealing to Ryūki that what he thought was her blood was only iron-rich hotspring water that she made appear red using a genjutsu. With his last resort having failed, Ryūki can only watch helplessly as Mirai defeats him with Wind Release: Whirlwind Fist. Ryūki and his followers are then turned over to Yugakure.

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