Flame Hashira Chronicles

The Generational Flame Hashira Record (歴代炎柱の書) is a book that chronicles the rich history of Flame Breathing. The records in the book have been diligently preserved and handed down through successive generations of the Rengoku family.

Overview
The book contains notes written by various Flame Hashira throughout history. The oldest notes date back to Sengoku period, written by a Rengoku ancestor Demon Slayer who was the first user of Flame Breathing. Passed down through centuries of generations of the Rengoku family, these records have been meticulously preserved until the Taishō era.

The latest edition of the book is owned by Shinjuro Rengoku and is titled 21st Generation of Flame Hashira (二十一代目炎柱ノ書). However, before retiring as the Flame Hashira, Shinjuro tore apart the book.

The book delves into the origins of Flame Breathing, revealing its roots in Sun Breathing, the original and foundational Breathing Style from which all others have derived. Sun Breathing is described as the very first technique ever created, holding a significant place in the history of Breathing Styles. The book strongly asserts that all subsequent styles are mere imitations of Sun Breathing, deemed blatant copies that are inferior in comparison. It is noted in the texts that practitioners of Sun Breathing traditionally adorn themselves with Hanafuda earrings, further signifying their connection to the ancient technique.

Additionally, the book documented a gripping account of the fateful battle between Yoriichi Tsugikuni and Demon Progenitor Muzan Kibutsuji. Yoriichi himself recounted the clash to the first Flame Breathing user, who diligently transcribed the account with great detail in the book. However, upon learning the outcome of the encounter, the Rengoku ancestor writes that his confidence faltered, feeling discouraged about the capacity of the Demon Slayers to defeat Muzan in the future. The narrative within the book also mentions Yoriichi's utilization of the thirteenth form of Sun Breathing during the confrontation.

Mugen Train Arc
When Tanjiro Kamado meets Kyojuro Rengoku on the Mugen Train, he inquires the Flame Hashira about Hinokami Kagura. However, Kyojuro expresses that he has never heard of it and swiftly changes the subject to something else. Later, as Kyojuro is nearing his final moments after his intense battle with Akaza, he advises Tanjiro to visit his family's estate, known as the Rengoku House. He mentions the possibility of finding some old notes left behind by older generations of Flame Hashira and remembers his father often reading them. Although Kyojuro himself never explored the contents, he speculates that there may be records related to Hinokami Kagura within those notes.

Kyojuro's Kasugai Crow, Kaname, showed Tanjiro the route to the Rengoku abode out of consideration for his deceased master's wishes. When he arrives, he meets Kyojuro's younger brother Senjuro Rengoku and his father Shinjuro Rengoku. The latter immediately recognizes Tanjiro as a Sun Breathing user, as he remembers the earrings being mentioned in the book. After Tanjiro had a scuffle with Shinjuro, Senjuro goes off to look for the book Tanjiro requested to have a look at, saying that he can guess which one as he sees Shinjuro with it most often. He returns with a book entitled "21st Generation of Flame Hashira". They sit down to open the book, but only discover that it's been ripped to shreds and almost unreadable. Tanjiro asks Senjuro if it's always been in this condition to which the latter replies that it definitely hasn't because it was supposed to have been preserved very well. He guesses that it was his father who tore it apart, and apologizes to Tanjiro, who tells him not to worry since it wasn't his fault. Senjuro feels bad that he came all this way only to discover that the book he'd wanted had been ruined, but Tanjiro once again tells him that it's fine since he can always try and discover more about the technique through training himself rather than reading. As Tanjiro prepares to leave, Senjuro says that he will try and restore the Flame Hashira book for him by using other sources and asking his father about its contents, saying that if he figures anything out he'll send a crow over to Tanjiro right away.

Infinity Castle Arc
While the final battle against the demons is raging in the Infinity Castle, in the Rengoku Estate, Senjuro prays to his late brother Kyojuro, pleading him to protect Tanjiro in the battle and thinks about a letter he wrote to Tanjiro. Senjuro had found some information written in the Generational Flame Hashira Record that might have been trivial, but could be useful to Tanjiro. It was a conversation between the Flame Hashira of that time and the Sun Breathing swordsman. It details about how the Sun Breathing user, the wielder of the first Breathing Style fought against Muzan at the time. Upon learning the story, the Flame Hashira ancestor of the Rengoku Family lost his confidence. Senjuro writes about the information in a letter he sends to Tanjiro through a Kasugai Crow, who arrived during Tanjiro's battle against Akaza in the Infinity Castle. Because the fight already began, before Tanjiro received the letter, he has to defeat Akaza in order to read it. The crow waits anxiously out of sight, watching the battle.

After Akaza's death, Tanjiro reads Senjuro's letter during a momentary break. After which, Tanjiro and Giyu Tomioka gets going again. While running, they get news about Genya Shinazugawa and Muichiro Tokito's death and Tanjiro thinks back to the letter he received from Senjuro. He understood from the description of the Sun Breathing user against Muzan that there is a thirteenth form of Sun Breathing, one more than the twelve Forms he currently knew. After Tanjiro sees the first Sun Breathing user perform the moves in a inherited memory of his ancestor Sumiyoshi Kamado, he deduces that the thirteenth form mentioned by the Flame Hashira in the notes can be produced by cycling through the twelve forms repeatedly.