Flame Hashira Chronicles

The Generational Flame Hashira Record (歴代炎柱の書) is a book that records the history of Flame Breathing and is passed down through generations of the Rengoku family.

Overview
The book contains notes written by different Flame Hashira throughout history, including those written by a Rengoku ancestor from the Sengoku period, who was the first user of the Flame Breathing style. The notes are passed down through a long line of generations of the Rengoku family. The book that Shinjuro Rengoku owns is the latest edition, titled 21st Generation of Flame Hashira (二十一代目炎柱ノ書). Before retiring as the Flame Hashira, Shinjuro tore apart the book.

The notes in the book includes information about Sun Breathing, describing it as the original Breathing Style. The first one ever created and the one which every other Breathing Style is derived from. Every other style is merely an imitation of Sun Breathing. They are blatant copies and inferior versions. The book also mentions that Sun Breathing users wear Hanafuda earrings. It also includes a detailed description of Yoriichi Tsugikuni's fight with Muzan Kibutsuji that he retold to the Rengoku ancestor, who wrote it down and got discouraged after hearing about the outcome. The description mentions Yoriichi's usage of the thirteenth form of Sun Breathing during the fight.

Mugen Train Arc
When Tanjiro Kamado meets Kyojuro Rengoku in the Mugen Train, he inquires him about Hinokami Kagura, but he admits that he has never heard of it before and switches to another topic. Later, as Kyojuro is dying after his fight with Akaza, he tells Tanjiro he could try going to his parent's home, the Rengoku Estate. He believes there are some notes, which the Flame Hashira of old left behind, saying that his father often read those, but he himself never did so he doesn't know what's inside them. But he believes there may be some record about Hinokami Kagura.

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.