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Robust Blade (強 (きょう) 靱 (じん) な刃 (やいば) , Kyōjin'na yaiba^?) is the thirty-fourth chapter of Koyoharu Gotōge's Kimetsu no Yaiba.

Summary

The battle continues with Zenitsu facing off against the Son Spider Demon, as Zenitsu continuously evades the attacks, he begins to feel symptoms from the poison. He collapses on the floor as the swarm of spider slaves quickly approach him.

He remembers a time in his past when he was berated by another unnamed student of Gramps. The boy angrily scolds Zenitsu for crying like a wimp and wonders why he was still being taught the Thunder Breathing techniques. When Zenitsu tries to interject and explain Gramps' reasoning, the boy throws a peach in his face and tells him to stop calling their Sensei "Gramps" and to get lost, ending his flashback.

Zenitsu reminisces on his thoughts of how he was never expected to become anything meaningful, or be useful in impacting in anyone's life, but when he met Gramps, he was the only one who believed in Zenitsu's potential and never gave up on him.

The Son Demon suddenly commands his slaves inject more poison into Zenitsu to speed up the process. He deduces that because Zenitsu has been running around so much, he would probably die before becoming a spider slave. However right before the slaves swarm Zenitsu, he grips his blade and emits a force which causes the air to vibrate. Sensing he is in immediate danger, the Son Demon retreats into the floating Cabin and spews his poison down onto Zenitsu. Before it can reach him, Zenitsu uses Thunder Breathing: First Form - Thunderflash Sixfold and ricochets off the trees before crashing through the cabin and decapitating the Son Demon.

Zenitsu drops from the air and crash lands onto the cabin, and begins falling unconscious. He starts to think of having happy dreams where he is strong and protects the weak, and realizes that his training was not a waste of time since he was able to make an impact on peoples lives. He comes to accept his fate but his mind advises him not to give up yet, telling him to use his Breathing Techniques to slow the poison from spreading. He thinks of how Gramps would be angry and how Tanjiro would be disappointed in him if he gave up easily, then Zenitsu finally passes out.

Elsewhere on the Mountain, Tanjiro asks Inosuke if he heard the sound of lightning in the distance, and how he smells the faint scent of thunder clouds alongside a foul smell. Inosuke angrily lets out a yell and tries to cross the river in an attempt to ignore Tanjiro, but is told to head down the mountain due to his injuries which angers him even more.

Tanjiro glances across the river and notices the Daughter Spider Demon standing on the other side of the river bank. She starts to run back into the forest and Inosuke tries to give chase after her, but she calls out to "Father" for help. Suddenly the large Father Demon unexpectedly drops in front of the two boys from above. The Father Demon angrily yells that they "STAY AWAY FROM [his] FAMILY!" As the Father Demon punching into the ground, he creates a force which blows the two boys away.

Characters in Order of Appearance

Events

• Zenitsu defeats the Son Spider Demon.
• Tanjiro and Inosuke chase after the Daughter Spider Demon, before encountering the Father Spider Demon.