Zenitsu Agatsuma vs Spider Demon (Son)

Zenitsu Agatsuma vs Spider Demon (Son) was a battle that took place on Mount Natagumo. It focuses on the battle between Demon Slayer Zenitsu Agatsuma against Son of the Spider Family.

Prologue
After recovering from his battle in the Tsuzumi Mansion and protecting Nezuko Kamado from the stranger Inosuke Hashibira, Zenitsu is directed to a new mission in the forests of Mount Natagumo, along with Tanjiro Kamado and the newly-joined Inosuke. However, as they reach the edge of the woods by nightfall, Zenitsu becomes unnerved and frightened, thanks to his enhanced hearing and heightened perception. He becomes even more reluctant when the three watch a fellow Demon Slayer being abducted by something and pulled into the forest. Deciding to stay behind, he sits on the dirt path as Tanjiro and Inosuke head on into Mount Natagumo. As he sits by himself alone, his sparrow Chuntaro attempts to push him to continue into the woods. Though the bird's attempts fail, Zenitsu's realization that Nezuko is in the forests with Tanjiro eventually causes him to sprint into the woods to search for her.

Though fear grips him as he walks into Mount Natagumo, it is replaced by frustration as he soon finds himself lost, feebly calling to Nezuko, Tanjiro, and Inosuke. Unaware he is being followed, Zenitsu ventures deeper into the woods, feeling a prick on his hand and finding the forest emits a strong pungent odour. Eventually, something crawls from behind him; he turns around to yell at it but looks down to see a large spider with a human head.

Terrified, he sprints away, heading further into Mount Natagumo. As he yells his wish to be awaken from a perceived nightmare, he soon finds himself in a clearing, with a suspended house in the middle. Strewn through the tree tops are long strands of silk, with captured Demon Slayers hanging from them. Watching as they turn into the spiders he saw earlier, he is interrupted by the appearance of a giant spider from the house. "Son" of the Spider Family emerges, horrifying the Demon Slayer even more.

Battle
Zenitsu tries to run away but Son taunts him by stating that he has already been bitten; he looks down and sees a blister and bruising on his hand, a bite from a spider earlier. The demon tell him that the poison will first cause pain, then dizziness, followed by nausea. Eventually, he will lost consciousness and then transform into a spider as well. Taunting him over his fate, Son sends numerous spiders to attack him, to which Zenitsu responds by climbing up a tree. Son jokingly tells him not to worry, as his intelligence will be reduced as he transforms. Upset over his impending doom, the Demon Slayer rants that he has no one that cares for him, to which Son responds to by offering another dose of poison.

Zenitsu clings to the tree and reminisces on his past; all the events that led to him eventually meeting his master to become a Demon Slayer. Like now, he had climbed up a tree and refused to continue training due to his weak resolve, despite his desires to live up to his trainer's expectations. Suddenly, a lightning bolt had struck him, causing his hair to turn blonde and him to fall out. Zenitsu recalls that moment and laments that everything he has experienced had led up to his transformation into a spider. As Son watches his strange opponent with curiosity and confusion, clumps of hair rip off of Zenitsu's head, showing that the poison is slowly affecting him. As the spider demons climb up to take him, Zenitsu suddenly loses consciousness and falls off the branch he was on, to the Spider Demon's laughter. Unable to believe he was sent to kill his family, the demon's pride disappears when Zenitsu begins to breathe, uprighting his body and planting his feet to the tree. He launches himself at Son, who retaliates with Poison Sputum, spitting a blob of acidic poison.

Shocked by his change in demeanour, Son continues to launch his poison at the Demon Slayer, where he soon realizes from his continual sole stance that Zenitsu is only capable of using one form. As time ticks down and the poison continues to affect his body, Son mocks his opponent as a Demon Slayer for only being able to do one thing. Zenitsu then recalls his master's reaction to him only able to do one form; encouraging him to hone it to perfection. He remembers that despite the humiliation and pain throughout his entire life, his master's encouragement had always given him the drive to keep moving forward. Son then signals for his spiders to envenomate Zenitsu with more poison; using their barbed tongue, they try to stab Zenitsu, who successfully dodges in time but suddenly coughs blood and stops, allowing the demons to overwhelm him. Landing to the ground and watching his minions attack with delight, he suddenly sees the spiders blown away as lightning seemingly envelops Zenitsu, frightening the Spider Demon. Zenitsu uses Thunder Breathing, First Form: Thunderclap and Flash and extends it Sixfold. Son desperate tries to launch a Poison Sputum blob but the poison doesn't reach Zenitsu. As he speeds around the clearing to look for a way to attack, Son cowers in fear and retreats back into his house. Seeing his chance, Zenitsu launches himself at the house, uses a thread to propel himself at the demon and decapitates Son instantly.

Aftermath
As his head fall and he begins to fade into ash, Son curses Zenitsu for how he had managed to defeat him despite being severely weakened by the poison's effects, before finally dying. Zenitsu lands atop the suspended house, realizing his dream to get stronger and help people has come true. Chuntaro lands down and tears up at the sight of his owner so badly wounded; Zenitsu attempts to apologize to his companion but the sparrow flies away. Using his breathing, he attempts to slow down the poison's circulation.

Eventually, after some time, Zenitsu is found by the Insect Hashira Shinobu Kocho, who had been tasked with aiding the Natagumo mission when the Corps had learned about numerous deaths among the Demon Slayers. Commending him for slowing the poison, she injects him with an antidote that cures him of the poison, though his arms and legs have shrivelled as a result of his ordeal. Soon, he is taken down the house by the Kakushi, patched up, and taken to the Butterfly Mansion to heal.