Shimane Arc

Covering volume 23 onward and chapters 201 to 211, this twelfth arc is the third entry in the Ultimate Yokai Saga, slotted between the Typhoon Human Arc and the Six Curses Arc. When Seiko's party reaches Shimane, two affiliates of the Serpo, the Reptilian and Red Baron, spring a trap meant to rob them of their powers. With only a narrow window left to change Momo, Unji, and Daiki back using the Uchide-no-Kozuchi, the group must put down these new foes before the opportunity vanishes.

Over a meal in Queen's office, Manjiro briefs Okarun that an unknown party is running a contest to harvest yokai powers with kozuka knives, a case his exorcist guild is already chasing. He also hands Okarun a twig he calls the Oni Club. The group then taxis toward Izumo Taisha, only for a priest to report that the sacred Uchide-no-Kozuchi has gone missing, spirited away by a so-called Phantom Ninja Thief. A tracker leads them to Matsue Castle, where Seiko, Momo, Bamora, and the Evil Eye give chase.

Inside the castle the priest proves to be bait, surrounding them with ninjas and revealing a Reptilian partner, the scheme built to steal their powers. Seiko sends Bamora and Momo down to a lower floor and faces the false priest alone, who is in truth Red Baron, a yokai of living smoke that her blows pass straight through. Once the Evil Eye arrives and falters too, Jiji takes the reins, landing hits with his Evil Gun by applying Seiko's Spiral Power and molecular chirality against the smoke. Mixing rapid control swaps with the Evil Eye into a new form, he finally hammers Red Baron into a pond.

Below, Bamora's invisibility and Momo's aura sense let them fell the Reptilian, but the alien resurfaces by transforming the entire castle into his giant body crawling with kite-borne ninjas. Choosing Seiko's life over her own cure, Momo orders Jiji to rush her wounded grandmother to a hospital, even knowing everyone will forget her. She and Bamora go kaiju, endure a fireworks barrage that drains the suit's endurance gauge, then plant the ninjas' explosives inside the Reptilian and detonate them with a Moe Moe Tri-Beam, blowing him apart.

Alone in a burning field as her curse peaks, Momo crumbles into fairy dust and vanishes. A procession of masked priests gathers her up, carries her to a hidden cave shrine, and taps the Uchide-no-Kozuchi to undo the curse and return her to full size. She wakes with no memory of aliens or of Jiji, stunning Seiko and Jiji at the hospital. Elsewhere, Saint-Germain bakes at Tasty Geezer Breads as Red Baron hands back the kozuka knives, and Seiko warns Jiji that Red Baron belongs to the Black Paladins, a clan of power-collecting diviners whose assassin, the Flying Ace, he matches.

Manjiro is revealed as the medium Queen once consulted, now living in a rented shrine and training Okarun in ki, while disclosing that he and Seiko's exorcist guild are tracking the power collectors. More of Saint-Germain surfaces: he moonlights as a baker, employs the former Kur affiliate, commands the smoke yokai Red Baron, and has struck an uneasy pact with the Serpo to seize Momo's group's powers for his Ultimate Yokai, pivoting to befriend Okarun after the forceful attempts fail. Red Baron stands exposed as the figure steering the power collectors once Saint-Germain lends him the kozuka knives, and Seiko names him a Black Paladins assassin, a faction the Masked Priests later describe as thirteen noble families vying to rule the world, with the Delacroix Family currently invading Japan under their prince Vlad. Reptilian joins as the Serpo's fifth subordinate, Jiji debuts a form blending his power with the Evil Eye's, and Momo's curse is finally broken, though doing so wipes away her memories, the ritual secretly infiltrated by Saint-Germain's shapeshifting affiliate Shinobi to steal her power.