Onigumo's cave

was a secret cave where the bandit, Onigumo was nursed by Kikyō before he fused with demons to become Naraku. The cave was set on fire a few days after Kikyō's death. Naraku's demonic aura still lingered in the cave 50 years after he was created. It was strong enough that not even grass grew where it permeated from.

When Onigumo was reborn as Musō, he had forgotten who he was. He ventured towards the cave where he began to remember his past.

Manga vs. Anime

 * In the manga, when Inuyasha, Miroku, and Kaede first visit the cave together, they encounter an illusion of Kikyō that was created by Naraku. She is covered in blood and tells Inuyasha that it is his fault that she died. The illusion is broken by Miroku when he presses his staff on top of a lizard. He explains that the illusion was created by incense and placed inside the lizard's stomach.
 * In one hour anime special "The Tragic Love Song of Destiny," a small spider yōkai is seen spying on Onigumo in the cave and later approaches him before he merges with many yōkai to become Naraku.