Onigumo's cave

was a secret cave where the bandit, Onigumo was nursed by Kikyō before he fused with demons to become Naraku. It was surrounded by a field of tall grass and had a tree growing on top of it. It sloped downward inside by its entrance.

History
Onigumo had taken residence in the cave after being terribly injured. He was secretly being cared for by Kikyō with only her sister Kaede knowing about it. Kaede would sometimes tend to Onigumo at the cave when Kikyō was unable to. He soon bargained with many demons for the ability to move again in exchange for them feasting on his flesh. This led to the creation of the hanyō 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 and his desire for Kikyō.

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 the one hour anime special "The Tragic Love Song of Destiny," a small spider yōkai is seen in the cave who told Onigumo that it's spying on Inuyasha that he's in love with Kikyō and later approaches him before he merges with many yōkai to become Naraku.