Magatsuhi is the evil half of the Shikon Jewel, born from the evil will of the demon that killed Midoriko, and one of the secondary antagonists of Inuyasha: The Final Act.
He was voiced by Takeshi Kusao in the Japanese version of the anime and Micheal Dobson in the English version, the latter of whom also voices Hoshiyomi.
When the Shikon Jewel was created from the souls of Midoriko and the demon she was fighting, this resulted in the creation of Magatsuhi, the evil spirit of the Jewel. It was he who sealed Kagome's spiritual power.Naraku summoned Magatsuhi from inside the Jewel, giving him a body made from his flesh. Both were united in wanting the Jewel to be corrupted. He first attacked Sesshomaru, who wielded Tenseiga, the only weapon that could kill him, as he is a spirit. He easily corrupted Kohaku's jewel shard. Sesshomaru bit his head off in dog form, but he continued to control the various parts of his body, as any damage to it didn't affect his spirit form. He also further sealed Kagome's power preventing her from purifying Kohaku's jewel shard. During the fight, Sesshomaru was able to put aside his obsession with Tetsusaiga, gaining his own sword, Bakusaiga, which he used to completely destroy Magatsuhi's body. Magatsuhi escaped in his true spirit form.
While Byakuya tricked Sesshomaru with an illusion of Magatsuhi, the real Magatsuhi possessed Kohaku. Miroku sucked Magatsuhi in with Wind Tunnel (without doing the same to Kohaku), but Magatsuhi survived (in a first for the series) due to receiving infinite power from Kohaku's corrupted Jewel Shard. Miroku was severely poisoned from sucking in his miasma. Still in Kohaku's body, Magatsuhi injured Sango and made his way to Naraku to give him his jewel shard. During the battle, Kohaku resisted Magatsuhi's possession, and Kikyo's light moved from the Shikon Jewel to Kohaku's shard, driving Magatsuhi out. Sesshomaru slashed him with Tenseiga, supposedly killing him, though his voice could be heard swearing revenge.
Magatsuhi survived due to part of him being inside Miroku. He left Miroku to possess Rin, and went to Naraku, who absorbed the completed Shikon Jewel to transform into a giant spider, with Rin inside him. At some time Magatsuhi left Rin but continued to stay with her. When Rin met up with Inuyasha, who had been turned into his demon form by the corrupted Shikon Jewel, Magatsuhi possessed Inuyasha and fought Sesshomaru. Through his love for Kagome, Inuyasha resisted his possession. Magatsuhi started to possess Kagome, but Inuyasha's dragon-scaled Tetsusaiga kept Magatsuhi frozen in place unable to possess anyone, and Sesshomaru slashed him with Tenseiga, destroying him for good.
His death broke the seal over Kagome, and allowed the light to return to the Shikon Jewel.
His incorporeal form is that of a disembodied pale face with red eyes and sclera and blue stripes on his cheeks. When he manifested in the living world through Naraku's flesh, he assumed the form of an adult male with long, shaggy white hair similar to Inuyasha's, smaller facial markings, blue lips and wore a set of dark armor. His arms were of a pale purple colored and most of the time, were a set of three sharp tendrils. As the body did not truly belong to him, he could deconstruct it back into the parts he borrowed from Naraku, keeping only the head in its form so he can speak.
- Magatsuhi is often mistaken to be the demon that killed Midoriko and whose soul continues to fight her inside the Shikon Jewel. This could be because the "two spirits of the Shikon Jewel" that Kaede spoke of are misinterpreted to mean the two souls sealed inside it, Midoriko and the Jewel's Demon, as well as Miroku's comment about Magatsuhi being created from the souls of the demons inside the Shikon Jewel, which many have interpreted as him being a fusion of the demons, when in reality he was created from their evil as part of the Jewel, as a separate entity from them. His true, spirit form does not resemble the dragonlike Jewel's Demon at all, and Kaede identifies his good counterpart as being called Naohi, not Midoriko.
- Thus, he is also mistaken to be the ultimate, final antagonist of the series, when at most he is a powerful ally of Naraku.
- Magatsuhi is the only enemy that has ever survived being sucked into Miroku's Wind Tunnel. This is something previously unseen in other invincible enemies of the series, such as Kaguya or Hakudoshi.