Gakusanjin

is a rock yōkai in the form of a mountain who owns the Fuyōheki, which conceals his energy and lets him sleep eternally.

History
Naraku stole the Fuyōheki from Gakusanjin to conceal his heart. Gakusanjin then helped Inuyasha and his gang to find Naraku, giving them a demonic aura crystal: if the aura disappears, the Fuyōheki will be near (They promised to return the Fuyōheki in return).

He was later killed by the Gyōja because they were not able to make a distinction between evil demons and innocuous demons.

The corpse of Gakusanjin was discovered by the group of Inuyasha. Afterwards it turns out that Goryōmaru deposited Gakusanjin's carcass in a cave to create a strong concentration of demonic aura.

Media appearances
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Manga

 * Chapter 335
 * Chapter 336
 * Chapter 346
 * Chapter 347
 * Chapter 348
 * Chapter 351

Anime
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 * Episode 165
 * Episode 166

Gakusanjin Gakusanjin 岳山人