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Mount Hakurei (白霊山, Hakurei-zan, "White Spirit Mountain") 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, and as the location where Naraku most significantly altered his body.
Hakushin, a once revered and great monk attempting to reach Nirvana by 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 in order to save themselves. With Hakushin's spirit in misery his soul remained behind after he died, unable to rest in peace. After many years, Naraku approached Hakushin 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 impure humans alike. Naraku used Mount Hakurei as his new base of operations while he constructed a more powerful body since most yōkai cannot enter such a pure place. In the anime, this is also where the Kon Blade of the Naginata of Kenkon was sealed away.
Rasetsu, (who is not given a name or back-story in the original manga) asked Kikyō to take a lock of his hair to "a sacred place where anyone can find salvation" This was his final wish. This is what lures Kikyō to Mount Hakurei, where Naraku inflicts a near fatal wound to her. In the Anime, Rasetsu is revealed to have a connection with Onigumo, and Naraku implies that he used Rasetsu in order to lure Kikyō to the mountain.
In order to buy time for himself to rebuild his body, Naraku used sacred jewel shards to resurrect the Band of Seven, a squad of seven mercenaries who were infamous for their mercilessness in combat. Naraku manipulates the Band of seven by promising that if they kill Inuyasha's group, Kōga, and Sesshōmaru, they will be allowed to keep their sacred jewel shards and keep on living. Although all seven of the band are eventually defeated, some of the members would occasionally gain the upper hand because as humans, they were able to enter the barrier of Mount Hakurei, with the only side affect being nausea, whereas Inuyasha, Kōga, Sesshōmaru, and Kikyō could not step inside the barrier without being purified.
After Kikyō convinced Saint 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, rough appearance. He also made it part of his body, making the inside of the mountain his "belly". Afterwards, demons started to emerge from the caves now that they could escape without being purified. Then after that, when battling Inuyasha and Kōga, Naraku destroyed the mountain, having no further use for it.
Later on in the story, Naraku returned to Mount Hakurei to reabsorb Onigumo's heart, hoping to make sure that Kikyō would not be able to purify the Shikon no Tama because of Onigumo's negative feelings. The mountain's remains now appears as several vegetated spikes.
Barrier of PurificationEdit
Hakushin's barrier prevented demons from passing through, by purifying them. Before its destruction, the average demon who sought to approach Mount Hakurei became very sick as shown by Shippō, Jaken 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 nauseous. In the anime, Sango and Kagome tried to make him go into the barrier, but he quickly got away with Shippō and Kirara, saying they should get them far from the barrier. This prompts a joke from Inuyasha: "Just how impure do you think he is?" In the Anime as well, the barrier prevented lecherous intentions from occurring; Miroku constantly tried to grope Sango while exploring the mountain, but got stung by the barrier.
The barrier however, only purifies and does not kill a victim if purifying them is not enough to kill them. A hanyō in contact with the barrier would have their yōkai energy forcibly removed, leaving them in their human form. Leaving the barrier caused their powers and hanyō form to immediately return. Having their powers purified in this way was very painful, but obviously was not lethal to them. Once their powers were purified, they can move a lot easier, albeit without their superhuman abilities. Because Mount Hakurei only affects yōki, yōkai that do not have any demonic aura such as Kanna are able to move in and out freely.
The song of Mount Hakurei was sung by several children living in its shadow.
Pure has become impure, impure has now become pure
Good has now become bad, bad has become good
To live is to die, to die is to live
Saint Hakushin beneath the earth (grave)
Mount Hakurei will return to Hijiri (virtuous monk)
Kirei wa kitanai, kitanai wa kirei
Ī wa warui, warui wa ī
Ikiru wa shinuru, shinuru wa ikiru
Hakushin Shōnin tsuchi no naka
Hijiri modori no Hakureizan
This song was of particular interest to Kikyō, and she often meditated upon its irony. As a priestess in the body of a corpse, dead but alive, no matter how much good she did by helping people, she still fed upon the souls of the dead.