Robust Blade

As Gramps' words about mastering one thing echo in his mind, Zenitsu dodges another spray of poison that dissolves his haori, while the demon's spiders force him onto the defensive. The toxin begins to numb his hands and feet. A memory surfaces of a rival trainee who berated him for crying, mocked his presence, and hurled a peach at him, scornful that a former Hashira would waste time on such a hopeless student.

Zenitsu reflects that as an orphan, no one ever expected anything of him or believed he could amount to anything. No one except Gramps, who scolded him relentlessly, dragged him back whenever he tried to flee, and never abandoned him. When the demon orders his spiders to inject more poison with their barbed tongues, Zenitsu vomits blood as he evades, and the spiders pile onto him. Gripping his sword, he makes the air vibrate, throwing them off, then unleashes a new variant of his form, Thunderclap and Flash, Sixfold, closing the distance with six rapid dashes and beheading the demon in an instant.

Falling onto the suspended house, Zenitsu feels he has fulfilled his wish to protect people and knows his time with Gramps was never wasted. Fighting the poison, he uses his breathing to slow its spread, refusing to disappoint his master or Tanjiro. Across the mountain, Tanjiro hears the lightning of Zenitsu's attack but is puzzled, since he smells no storm. A pale young demon then leaps across the river, and when Inosuke gives chase, she calls for her father. A giant humanoid Spider Demon emerges, demanding the Slayers leave his family alone, and his punch against the riverbed hurls both boys back.

Zenitsu defeats the Son of the Spider Family using a self-taught expansion of his only technique, proving his master's faith in him. The chapter concludes their battle.

Tanjiro and Inosuke discover the Daughter of the Spider Family, who resembles Mother and Rui, and she summons Father. The encounter begins the fight against the Father of the Spider Family.

This Mount Natagumo Arc chapter was adapted in Episode 17 of the anime. It delivers Zenitsu's first major victory and showcases the depth of his bond with Jigoro Kuwajima.

The chapter also marks the introduction of the Father of the Spider Family as a formidable physical threat, setting up the multi-front struggle on the mountain.