Mount Hakurei

was a once sacred mountain of absolute purity and divinity. Beautiful flowers grew from the foothills below to the mountain's summit, and it was a place where no yōkai dared to enter. This mountain served as the battleground for the one of the most prominent battles of the InuYasha franchise, and the only battle of the series where all 12 main characters played a part.

History
Hakushin, a once revered and great monk fell into despair while attempting to reach Nirvana, becoming a living Buddha as he died. Near death, the Monk was filled with regret, having spent his entire life helping others, and the villagers he healed and guided gladly let him die to save themselves. With Hakushin's spirit in misery and unable to rest in peace, Naraku approached him and spoke words that Hakushin was too cowardly to admit to himself. Manipulated in his vulnerable state, the monk agreed to help him, and with his new powers, was able to manifest an impenetrable barrier that could ward off demons and humans alike. Naraku used Mount Hakurei as his new base of operations while he constructed a more powerful body. In the anime, this is also where the Kon Blade of the Naginata of Kenkon was sealed away.

Rasetsu also asked Kikyo to take his lock of hair to "a sacred place where anyone can find salvation" as his final wish. This is what lures Kikyo to Mt. Hakurei, where Naraku inflicts a near fatal wound to her.

Barrier of Purification
Hakushin's barrier prevented demons from passing through, by purifying them. Before its destruction, the average demon who sought to approach Mt. Hakurei became very sick as shown by Shippo and Kirara, and the closer one got, they would react in physical pain, with purifying energy crackling over their body and slowing them down. Only very powerful demons such as Sesshōmaru have some resistance to being very close to the barrier, and even then he was somewhat weakened. Any item that has yōkai abilities, while not destroyed like demons are, lose those abilities. If someone was affected with something like demonic poison (or perhaps even regular poison), it would also be purified.

Not only were demons affected but at least some humans were as well. Most humans could feel the peace and serenity the barrier caused the surrounding area. However, the evil undead human Band of Seven did not like being in or around the barrier for the way it gave them an unwell feeling due to be being brought back from the dead and their murderous natures. Even Miroku was affected by the barrier, making him feel nauscious. In the anime, Sango and Kagome tried to make him go into the barrier, but he quickly got away with Shippo and Kirara, saying they should get the, far from, the barrier.

A Hanyō in contact with the barrier would have their yōkai energy forcibly removed, leaving them in their human form. Leaving the barrier causes their powers and hanyō form to immediately return. Having their powers purified in this way is very painful, but obviously is not lethal to them.

Although no Yokai can pass through the barrier, there seems to be an exception for Yokai that do not have any demonic aura such as Kanna.

Destruction
After Kikyo convinced St. Hakushin to dissolve the barrier, Naraku's miasma corrupted the mountain, turning its rich vegetation into a barren wasteland and making it take a very jagged, roughappearance. He also made it part of his body, making the inside of the mountain his "belly". Afterwards, demons started to emerge from the caves that were unaffected by the barrier. Then after that, when battling InuYasha and Koga, with a single attack Naraku destroyed the mountain of purity.

In Later Chapters
Later on in the story Naraku returns here to reabsorb Onigumo's heart, hoping to make sure Kikyo cannot purify the Shikon no Tama because of Onigumo's negative feelings. The mountain's remains now appear as several vegitated spikes.

Song
The song of Mount Hakurei is sung by several children living in its shadow.

Kirei wa kitanai, kitanai wa kirei

Pure is impure, impure is pure

Ii wa warui, warui wa ii

Good is bad, bad is good

Ikiru wa shinuru, shinuru wa ikiru

To live is to die, to die is to live

Hakushin Shounin tsuchi no naka

Hijiri modori no Hakureizan

Hakushin Shounin within the earth (grave)

Mount Hakurei will return to Hijiri (virtuous monk)

This song is of particular interest to Kikyo, and she often meditates upon its irony. As a priestess in the body of a corpse, dead but alive, no matter how much good she does by helping people, she still feeds upon the souls of the dead.