WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Vol. 12, by Koyoharu Gotouge, John Werry and John Hunt, available in English from Viz Media.
The latest translated manga volume of Demon Slayer: Kimetsu no Yaiba, Volume 12, which collects Chapters 98 to 106 of the manga, focuses on Muzan looking to strike back at the Demon Slayer Corps following the defeat of Gyutaro and Daki, who shared the Upper Rank 6 title. The volume is quite important as it introduces us to both the secret village that houses the swordsmiths who forge the Demon Slayers' Nichirin blades, as well as Muzan's Upper-Rank members.
This collection opens with Muzan holding the first meeting of the Upper Ranks, also known as the Upper Moons, in over 113 years to announce that they are now incomplete. This meeting to discuss the next steps is also the best glimpse so far at Muzan's top generals, as most members make their manga debut. The King of Demons then sends Gyokko and Hantengu, the 5th and 4th Upper Ranks, respectively, to investigate a new lead to strike against the Demon Slayer.
Meanwhile, Tanjiro awakens from being unconscious for two months after his fight with Gyutaro and Daki. During that time, he has a dream, later described as an "inherited memory," from the perspective of his ancestor Sumiyoshi, who has a conversation with an unnamed swordsman. While he was asleep, Tanjiro also received a letter from Hotaru Haganezuka, the swordsmith who crafts his blades, and voiced his frustration at having another weapon damaged.
Desperate to make amends and apologize to Haganezuka for chipping his blade, Tanjiro receives permission to visit the Swordsmiths' Village. Due to the secret village location, it requires several safety precautions to prevent anyone from learning of its position. After arriving, Tanjiro visits the hot springs since no one knows where Haganezuka is. There, he runs into Genya Shinazugawa, the younger brother of the Wind Hashira, who immediately storms off, but not before Tanjiro notices Genya is missing a tooth.
Tanjiro also encounters the Love Hashira, Mitsuri Kanroji, who tells him that she is "rooting for the Kamado siblings." Before she leaves, she also mentions that there is a secret weapon in the village that can make you stronger. Naturally, this inspires Tanjiro to go on the hunt, but instead of a secret weapon, he first finds yet another Hashira, Muichiro Tokito.
We're then introduced to Kotetsu, a young boy from the Swordsmith Village, who has a key that the Mist Hashira is trying to get as it unlocks the secret weapon Kanroji mentioned. Tokito states that they're having a pointless conversation, believing that a Hashira's time is more valuable than a swordsmith's and tries to take the key by force. However, Tanjiro intervenes, angrily arguing that swordsmen and swordsmiths have a symbiotic relationship and need each other, which results in Tokito knocking him out.
Tanjiro awakens to a thankful Kotetsu, but the boy reveals he gave Tokito the key, which activates a 300-year-old mechanical doll called "Yoriichi Type Zero." Kotetsu's ancestors modeled it after a swordsman from the Sengoku period and made it with six arms to recreate their skillful battle movement. Since Kotetsu is the only surviving member of his family, he's afraid that if it breaks, his inability to fix it would shame their legacy.
Tanjiro can't help but admire the Mist Hashira's skill, with Tokito's crow even mentioning that he is a descendent of the users of Sun Breathing, the original style of breathing. Tanjiro then recognizes the doll's face as the unnamed swordsman from his dream. It's Kotetsu that introduces him to the idea of inherited memories, through which someone can see their ancestor's memory.
Tokito stops training after breaking one of the Yoriichi Type Zero's arms and part of its armor, which sends Kotetsu running away distraught. However, Tanjiro tells the boy he needs to put an effort forward and that they can work hard together. He notes that even if he fails to save Nezuko and kill Muzan, he knows that the people saved along the way will eventually achieve that goal. Motivated by Tanjiro's words and uplifted by the fact that the doll still works, Kotetsu is ready to help Tanjiro get stronger.
Kotetsu's anger towards Tokito reawakens his vigor and eliminates his doubts over being able to repair the doll, even using his eye for analyzing fighting styles to help Tanjiro train. While he initially struggles to evade the attacks of the doll, Tanjiro is eventually able to awaken something similar to his "opening thread," which helps him predict the movement of strikes. After he successfully lands a forceful blow on the Yoriichi doll, its head breaks open and reveals an old katana within the body.
Tanjiro and Kotetsu inspect the blade and notice it's tarnished due to lack of care over the centuries. Before they can decide what to do with it, a now insanely muscular Haganezuka emerges from the woods and keeps repeating, "Leave it to me," while trying to take the sword. Kanamori, another swordsmith, also appears to tell them that Haganezuka has been training in the woods so he can create better blades for Tanjiro, so they let him take the old katana.
As Tanjiro waits for his new blade, he again tries to break through Genya's abrasive exterior but is again unsuccessful. We get our first indication into some of Genya's abilities as Tanjiro notices that his tooth has now grown back. Though he denies having lost it in the first place, this shows that he likely has a type of regenerative ability, but before Tanjiro can ask any more about it, Genya kicks him out.
This leads to the climax of Vol. 12, in which Gyokko and Hantengu infiltrate the Swordsmith Village. Just as Tokito and Tanjiro are about to look for Kanamori and Haganezuka to check on their new swords, they encounter Hantengu. Acting quickly, Tokito uses Mist Breathing to cut off his head, but instead of dying, he spawns into two different demons, Sekido and Karaku.
Tokito is sent flying by a powerful gust of wind from Karaku while Sekido's staff electrocutes Tanjiro. As the electrical shock begins to cause Tanjiro to pass out, he notices Genya behind the demon with a gun aimed at it, setting the stage for another significant battle against the Upper-Rank demons in the next installment.