Tanjiro, Zenitsu, and Inosuke go on their first mission together and venture to Mount Natagumo.
Tropes associated with Natagumo Mountain Arc:
- Arc Villain: Rui, the leader of the Spider-Demon Family who occupies Mt. Natagumo and is also a member of the Twelve Kizuki, being the Lower-Rank Five demon.
- And I Must Scream: How the Older Brother Spider Demon turns humans into his puppets.
- Big Damn Heroes: Giyuu intervenes and saves Tanjiro just before Rui can kill him.
- Blood Knight: Inosuke, natch. He not only rushed into a fight with the Father Spider Demon who kicked his ass, but Giyuu as well, who killed the Spider Demon, and in other words much above his league.
- The Bully: Kaigaku doesn't leave any stones unturned denouncing Zenitsu as weak and a coward in the flashbacks.
- Cool Old Guy: Jigoro Kuwajima, the retired Thunder Hashira and Zenitsu (and Kaigaku)'s trainer.
- Cliffhanger: The arc ends with Tanjiro and Nezuko captured, and stated to put a 'trial' regarding Nezuko's future status.
- Curb-Stomp Battle:
- Inosuke and Tanjiro vs Father Spider Demon has the latter come out on top, and then Tomioka shows up and immediately turns the tables.
- Tanjiro and Nezuko vs Rui.
- Both Father Spider Demon and Rui vs Giyuu.
- Darkest Hour: Tanjiro's Hinokami Kagura fails, Rui re-attaches his head without any trouble, Tanjiro has to fight him with his broken sword and Rui has captured Nezuko, with intent to make her a Spider Demon and cut all her ties to Tanjiro.
- Despair Event Horizon: Rui's parents hit it when their son killed one of their acquaintances under influence of his Horror Hunger. They planned to kill Rui and then kill themselves in order to repent.
- Domestic Abuse: The Father Spider Demon tortured the Mother Spider Demon while Rui watched.
- Evil Smells Bad: The Older Brother Spider Demon. Zenitsu comments his smell is so bad Tanjiro would die just with a whiff.
- The Gadfly: Shinobu towards Giyuu.
- I Control My Minions Through...: Fear. Rui imposes his rule extremely viciously, and mere thought of rebellion is punished with death-by-sunlight.
- Mercy Kill: Tanjiro lets the Mother Spider Demon die without any pain using Fifth Form of Water Breathing.
- My God, What Have I Done?: Rui's reaction after killing his parents, and after reuniting with them after his own death.
- Oh, Crap!: The Older Brother, who has moments to register what happened before Sleeping Zenitsu cuts his head off.
- Playing with Fire: Nezuko's Blood Demon Art.
- Screw This, I'm Outta Here: The Sister Spider Demon tries to flee after the Spider Clan is killed, but she is intercepted by Shinobu and is poisoned by her when she stops to sadistically feed on Murata.
- Self-Made Orphan: Rui killed his parents and never realized it till his own death.
- Too Dumb to Live: One random Demon Slayer ends up walking in between Tanjiro and Rui's confrontation, arrogantly boasts that he can kill Rui effortlessly due to his size, and refuses to hear Tanjiro's warnings that this demon isn't somebody he can defeat easily. Rui just simply dices him as soon as he attempts to charge into battle without even bothering to look in his direction. Even if the Demon Slayer had no way of knowing that Rui was one of the Twelve Kizuki, trying to charge at an unknown enemy just because he doesn't look strong to you is a surefire way to getting yourself killed.
- The Worf Effect: The Father Spider Demon beats up both Inosuke and Tanjiro brutally, so bad that Tanjiro mistook him for the Lower Moon demon. Similar to him, Rui beats Tanjiro and Nezuko badly and nearly kills both of them. In both cases, Giyuu intervened and killed both the Father Spider Demon and Rui, showing the true power of a Hashira.
- You Are Already Dead: Eleventh Form of Water Breathing, Dead Calm. Rui didn't even notice he was decapitated until after he noticed why his Blood Demon Art wasn't working.
