The following contains spoilers for Chapter 14 of Naruto: Konoha's Story -- The Steam Ninja Scrolls: The Manga, "The Resurrection Hot Springs," by Masashi Kishimoto, Natsuo Sai, Sho Hinata, Nathan A. Collins and Brandon Bovia, now available in English from Viz Media.
One of the scariest techniques seen in the Naruto franchise has been the puppet jutsu. Many users have deployed it, subverting different styles to create puppets for war. Kankuro and Shinki of the Sand are prime examples, while Urashiki had his Ōtsutsuki puppets as well in the Boruto era. In these cases, chakra was both the connector and fuel.
Then there's Sasori of the Akatsuki, who became a puppet himself to control a fleet. This technique has given heroes from Konoha and other nations severe problems because the puppets are sturdier copies, who don't disappear like the shadow clones. Interestingly, the technique is made even deadlier when Naruto: Konoha's Story -- The Steam Ninja Scrolls has Ryuki unleashing his take on the move. He adds in a key twist that inhibits the heroes from attacking, making it a very clever adaptation.
Ryuki Subverts the Blood Puppet Technique in Naruto's Steam Ninja Scrolls
Ryuki is a cultist who follows the Jashin way. His idol is Hidan of the Akatsuki, who used a blood technique to cut his opponents and link his body to theirs. He turned himself into a voodoo doll, harming himself and killing the enemy by extension. Admittedly, it was slow and required a lot of trapping and planning, but once hooked in, it was effective as Hidan was immortal. Thus, he'd always recover.
Ryuki has evolved that concept in Chapters 13 and 14 of Steam Ninja Scrolls, using his own appropriation known as the Curse Technique: Body Controlling Manipulated Blood. He combines this with his scythe, via Bukijutsu, and after cutting Mirai, turns her into his puppet. Shockingly, he stabs the voodoo doll representing and controlling her, which kills the real Mirai -- or so readers think. Luckily, she used a genjutsu and didn't get cut after all, so she's able to break the hold and fight back.
Ryuki's Blood Jutsu Weaponizes Innocent Spectators
However, this technique would create a problem if Ryuki did succeed. After all, Kakashi and Might Guy would have had to face an enslaved Mirai, whether she was alive or as a corpse. Thus, Ryuki makes the concept of the puppet a living flesh horror show, even nodding to how the Edo Tensei resurrected people for masters to control. In the end, the heroes would have had to hurt or kill one of their own to succeed, so it's easy to see why Kakashi would be concerned.
While Mirai doesn't end up becoming this pawn, Ryuki pivots and uses the voodoo doll to turn the innocent Tatsumi into his puppet. After cutting Tatsumi, he has her attack Mirai, knowing the heroes wouldn't harm someone who's not a warrior. It's much faster than Hidan's ways and runs the risk of collateral damage. There's also a mental advantage, as he knows they wouldn't turn Tatsumi into a casualty.
This is maximum offense versus things like wooden dolls, clones or robot puppets, as the humans weaponized are victims who have no control. Thankfully, Mirai uses her remixed Rasengan to knock Tatsumi out, take down Ryuki and break the jutsu. But it leaves Kakashi and Guy wondering what other cults exist out there, reworking old moves to become better legacy villains. This Ryuki battle is ultimately a throwback to the past for Naruto nostalgics, but a stern warning of how dangerous the future still is.