The Tsuzumi Mansion Arc (鼓つづみ屋や敷しき編へん Tsuzumi Yashiki Hen?) is the fourth arc of Kimetsu no Yaiba.
Tanjiro ventures south-southeast where he meets a cowardly young man named Zenitsu Agatsuma. He’s a fellow survivor of the Final Selection, and his sparrow asks Tanjiro to help keep him on the straight and narrow.
Together, Tanjiro and Zenitsu venture into a mountain where they find a mysterious house. They meet Shoichi and Teruko, two small children whose brother has been kidnapped and taken into the house. The sound of a Tsuzumi echoes through the mansion from time to time, and it all belongs to a Demon.
Tanjiro and Zenitsu go to the house but are separated soon after, tasked with protecting one of the children each. Zenitsu accidentally discovers another young Demon Slayer who wears a boar hide as a mask. The boar Inosuke charges the house trying to kill the same demon as the others.
Inosuke and Tanjiro come to blows with the demon Tsuzumi who controls the house. They end up being transported away while Zenitsu and Shoichi are chased all over the house by the Tongue Demon. While sleeping, Zenitsu shows off his incredible ability to breathe thunder. At the same time, the boar demonstrates his self-taught breathing techniques against the horned demon.
Tanjiro and Teruko locate Kiyoshi and learn that the demons want to eat him because he possesses rare “marechi blood”. Tanjiro and Kyogai come face to face in a fight where Kyogai constantly changes the direction of the room using his Blood Demon Art. Kyogai is also revealed to be a former member of the Twelve Kizuki.
At first, Tanjiro struggles due to the injuries he suffered in Asakusa. After regaining his mind, Tanjiro is able to adapt to the changes by altering the form of his own aquatic techniques. He sees fit not to trample on Kyogai’s handwriting and even compliments his Blood Demon Art before chopping off his head. Kyogai dies peacefully, knowing that his talents have been recognized by someone.
Everyone regroups outside where Inosuke comes to blows with Tanjiro and Zenitsu for Nezuko’s box. Inosuke tries to kill the demon without understanding the situation. Although Zenitsu is in the same boat, he does his best to protect the box for Tanjiro. Tanjiro had mentioned earlier that this box was more precious than his own life.
Tanjiro and Inosuke engage in a fierce bare-knuckle fight that ends in a concussion that Tanjiro inflicts on the boar with his rock-hard head. The Demon Slayers and the Children part ways and the new trio recuperate at Wisteria House. There, they get to know each other and confront their personalities. Nezuko also reveals herself and Zenitsu learns the truth about why Tanjiro is traveling with a demon.