Zenitsu Agatsuma vs Older Brother Spider Demon was a battle that took place on Natagumo Mountain. It focuses on the battle between Demon Slayer Zenitsu Agatsuma against the Older Brother Spider Demon.
Zenitsu and Chuntaro journey together through Mt. Natagumo in search of Nezuko.
Zenitsu is constantly frightened by the spooky, dark, and forested areas. He even gets picked by something and starts to get annoyed he can't find Tanjiro or Inosuke. The loud demon slayer is also irritated by the foul odors, multiple pathways, and the sounds of spider's rustlings around his feet.
After hearing another noise behind him, Zenitsu turns around to tell it to stop and is shocked to see a Spider with a human face.
Spooked, Zenitsu runs away at full speed screaming for his life. He happens upon a small house suspended by spider threads. Caught in the webs are several humans who have been poisoned by spider venom.
A pungent odor fills the air and a giant spider with a demon's head lowers himself from the house. Zenitsu refuses to talk to the scary monster and flees. The Spider Demon tells him it's futile now that a spider has injected him with venom. The poison will change him into a spider in less than an hour. To terrify the weeping demon slayer, the demon walks him through the agonizing details of the transformation minute by minute.
A swarm of spiders with human faces crawl at Zenitsu, spooking the demon slayer into fleeing again. He runs up a tree even though the demon claims it's pointless. The cowardly boy tries to ignore the monster, so the Spider Demon threatens to give him an extra dose of poison so he deforms into a spider sooner.
Hopeless, Zenitsu attempts to hide from the demon. Zenitsu recalls his Master disciplining him for running away. That's how he got struck by lightning and ended up with yellow hair. Zenitsu wants to be more competent but he believes he's too weak and is trying his best. The demon is confused by his opponent's strange wailing and sends his minions crawling up the tree.
Zenitsu panics and realizes that not only is he in trouble, but his hair is falling out as well. He meets the Spider Demon's glare and passes out from the fear. Even more confused, the demon wonders if Zenitsu is really a demon slayer. The spiders reach the branch Zenitsu is laying on, but his body falls off. The demon believes Zenitsu is going to fall on his head and says he wouldn't have been useful anyway.
The sleeping hunter takes a deep breath, smoothly flips over, and assumes Thunder Breathing: First Form's stance. Zenitsu propels himself off the tree and soars toward his enemy. The Spider Demon counters with Poison Sputum, a blast of poison shaped like a web. Zenitsu twists his body in mid-air, impressing his opponent. Moving like a new man, Zenitsu takes his stance again.
The demon orders his spider minions to swarm his enemy, interrupting the demon slayer's stance. Zenitsu evades their attacks and assumes the stance of Thunderclap and Flash. The demon fires more poison at him but the sleepwalker jumps back to evade it and reassumes his stance. This makes the Spider Demon believe Zenitsu can only utilize a single technique in battle.
Zenitsu dreams of his master, who reminds him that he must master a single thing. All Zenitsu's hellish training pays off, his electric aura rises as he recalls his sensei's powerful words of encouragement. The old man was hard on Zenitsu because he wanted to beat out his impurities and forge him into the most durable blade of all.
The Spider Demon spews more venom and although Zenitsu dodges, he's surrounded by the swarm of spiders. Memories of Zenitsu's encounters with his Sensei's other pupil flash by. He also remembers that having no parents meant no one cared about him. However, his Sensei was always there to believe in him no matter what. Zenitsu manages to avoid the poison stingers from the spider's but coughs up some blood as a result of getting bitten earlier.
The largest swarm of spiders with human faces all jump on Zenitsu and appear to consume him. Suddenly, Zenitsu's electric aura explodes and sends all the spiders flying away. The sound of thunder echoes through the area and the air vibrates from the intensity resonating from the demon slayer. Determined to make his master proud, Zenitsu opens his eyes and assumes the stance one final time.
The Spider Demon lands on the ground and watches in fear as his opponent charges up Thunderclap and Flash. The lightning resonating from the Zenitsu breaks the ground and it grows even stronger when he multiplies its power Sixfold!
Afraid of Zenitsu's ferocious aura, the Spider Demon retreats upward and spits poison at the electrified hunter. Zenitsu vanishes in an instant and dashes around the surrounding area with lightning-speed. Unable to track his enemy's movements, the demon retreats back into the house. Zenitsu appears and propels himself off one of the wires. In a lightning-fast series of six dashes, Zenitsu follows the threads until he cleaves the demon's head off.
Zenitsu rises into the light of the moon while the demon laments literally losing its head. As he fades away, the demon can't come to terms with being killed by someone like Zenitsu.
The wounded demon slayer falls onto the suspended house. He believes its time to dream a place where he is strong and doesn't let anyone down. Struggling to withstand the poison, he also knows his time is up.
Sad, Chuntaro weeps and flies away to find help. Zenitsu's sensei's voice reminds him not to give up. He decides to use the breathing technique to slow down the venom's effects for as long as he can. His master would be upset if he took the easy way out, and so would Tanjiro.
A short while later, it gets difficult for Zenitsu to breathe and he losses feelings in all his limbs. He apologizes to Nezuko but remembers his master's words to never give up. The weary demon slayer opens his eyes to see a butterfly floating toward him. Turns out Chuntaro went and got Shinobu for help. The Hashira stands over Zenitsu and asks about his condition.