Demon Slayer: Kimetsu no Yaiba Episode 15: Mount Natagumo

Tanjiro, Zenitsu, and Inosuke fully recover at the old woman's house where they've been staying. Zenitsu learns that Nezuko, the girl in the box Tanjiro carries, is his sister and immediately becomes smitten with her, chasing her around playfully while Inosuke headbutts Tanjiro in a scuffle inside the house.

The doctor confirms their recovery, and a Kasugai crow arrives with orders for them to head northeast to Mount Natagumo to investigate. The group thanks the old woman for her hospitality; she gives Inosuke clean clothes, mentions tempura coated in flour which flusters him, and lights a flint to ward off evil spirits before wishing them luck.

As they approach the ominously looming, dark mountain, eerie creaking sounds and an acrid smell emerge; Tanjiro senses something and rushes ahead with Inosuke, leaving a scared and grumbling Zenitsu behind. They spot a wounded Demon Slayer on the path who pleads for help but is suddenly yanked back into the woods by thin threads attached to his back.

Deeper in the forest, Tanjiro and Inosuke encounter Murata, another Demon Slayer who's still alive and terrified; he explains that ten Slayers were sent to the mountain, but they suddenly turned on each other, killing most of the group one by one. Murata is upset that lower-ranked Slayers like them arrived instead of a Hashira. Inosuke punches him to demand details.

A strange creaking sound echoes, and the remaining controlled Demon Slayers—moving sluggishly and erratically with swords drawn—attack the trio. Tanjiro realizes they're being manipulated and subdues them non-lethally alongside Inosuke, who cuts the threads attached to their backs manipulated by small spiders.

Without Tanjiro's sense of smell detecting the demon clearly, Inosuke uses his super spatial awareness to track the source controlling the webs. A pale-skinned, white-haired demon with multiple eyes appears from the shadows, warning them not to interfere with his "family" or his "mother" will kill them; he walks away casually along a massive spider web high above.

Inosuke charges to attack but can't reach the web. Meanwhile, Zenitsu, furious that Tanjiro brought Nezuko into the dangerous forest, rushes after them.

Elsewhere, the Demon Slayer Corps leader—a man in a box-like structure—notes that many of his "children" have been slaughtered on the mountain and dispatches two Hashira, the Water Hashira Tomioka Giyu and another Hashira named Shinobu, suspecting involvement of the Twelve Kizuki.