Magatsuhi is the incredibly powerful yōkai in InuYasha who killed Midoriko and whose soul with whom she continuously battles inside the Shikon Jewel. Magatsuhi was created by numerous demons who formed into one demon in order to kill Midoriko.
He is the final antagonist of InuYasha: The Final Act and the indirect true and most dangerous antagonist of the entire franchise.
He is 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.
During the story
Magatsuhi is first seen in manga chapter 89 as the mummified corpse of an unnamed but massive demon. He is the demon who fought and died with the ancient priestess Midoriko hundreds of years ago.Later in the manga, the demon's name is revealed to be Magatsuhi, and Naraku helps him manifest in physical form so that Magatsuhi can help him complete the Shikon Jewel. Magatsuhi seems to fear Kagome's spiritual power and seals away her true strengths to prevent her from purifying him. Magatsuhi's first mission after coming to life is to defile and obtain Kohaku's shard but he is obstructed by Sesshōmaru, whose sword Magatsuhi fears, as it can kill him. Magatsuhi flees after being injured in the fight (having lost the flesh Naraku lent him), and along with Naraku tricks Sesshōmaru into leaving Kohaku alone with a fake Magatsuhi. He succeeds in kidnapping Kohaku, made unconscious by Magatsuhi's defilement of his shard, and severely injures Sango and Miroku who tried to stop the demon. He remained in control of Kohaku, tormenting the boy with visions of his past, until Kohaku managed to awaken and break free of Magatsuhi. Kohaku then attempted to drive the demon from his body, even at the cost of his own life. Magatsuhi curses him and fights back until being slashed by Sesshōmaru, who has returned after realizing he was tricked. Sesshōmaru attempts to kill Magatsuhi but he escapes at the last moment, having hidden his essence in Miroku, who he poisoned.
Magatsuhi then appears inside Naraku's enormous body in the final battle, possessing Inuyasha and trying to kill Rin and Kagome. Sesshōmaru mocks him, taunting Inuyasha was the worst person to possess and Magatsuhi finds Inuyasha fighting back at his control. He is driven from Inuyasha's body and after trying to take over Kagome, is apparently killed by Sesshōmaru.
However, Magatsuhi is not completely dead, as he is still connected to the Sacred Jewel, and his spirit is still in combat with Midoriko inside the Jewel. Inuyasha inadvertently dives into the Jewel and encounters a giant dragon-like demon. Possibly unknown to Inuyasha, this is the true Magatsuhi, still living and battling within the Jewel. Inuyasha slices him immediately with the Wind Scar, and he is split into the many demons that formed him. Magatsuhi states that Inuyasha does not need to keep searching for Kagome, and says that she'll be here soon enough. He also states that if Kagome were to make a selfish wish on the Shikon Jewel, Kagome would replace Midoriko in the eternal fight in the Jewel and Naraku would replace him.
Magatsuhi's true plan is finally realized - he manipulated Naraku through the Jewel in order to replace himself with Naraku, thus freeing himself from the Jewel in the process, leaving him free to unleash total destruction upon the world once again, with no one to stop him, revealing himself as the true mastermind of the series.
Inuyasha refuses to believe Magatsuhi, and proceeds to attack the demons that form him in an attempt to reach Kagome and save her. In the end he and Naraku were destroyed forever by Inuyasha when he cuts the point of light in the darkness inside the Shikon Jewel with the Meidō Zangetsuha and arrives in the Meidō outside the Jewel to save Kagome. Then Kagome makes the right wish; the wish being for the Jewel to be taken out of existence. The Jewel breaks and then disappears. Magatsuhi (the demons that form him) is enveloped in light, along with Midoriko and Naraku, finally putting an end to his, the Jewel's, and Naraku's evil. Inuyasha and Kagome are transported back to earthen ground.
By the end of the series, it is clear that Magatsuhi is indirectly responsible for almost everything terrible that happened in the whole story. Ultimately, he is the true and most dangerous villain, as he is the indirect reason the Shikon Jewel exists at all, having forced Midoriko to create it in her death, and so everything going wrong throughout the entire story, as a result of the many struggles for the Jewel, or the Jewel's own plans to keep the battles between good and evil going inside of it, ultimately leads back to him and his desire to destroy Midoriko.
While he is an amalgamation of demons forming an oriental sort of dragon residing in 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.