Magatsuhi

is the incredibly powerful yōkai 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. Magatsuhi's true form is much more monstrous.

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.

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 (along with the Jewel itself) 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.

Physical description
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.

Personality
As the embodiment of the darkness of the Shikon Jewel (and the indirect reason it exists), Magatsuhi is pure evil, perhaps even more so than Naraku himself. He views every other single being, living or not, as inferior to him, repeatedly calling Sesshōmaru weak and disliking any comparisons of him to Naraku. He sneers at courage and self-sacrifice as seen when badly injuring Miroku and Sango, taunting Miroku for absorbing Magatsuhi's deathly poison to save Kohaku from him and sneering that he will die. He is also sadistic, promising Sango he will return her brother as a corpse once he is through with him.

Powers & Abilities
Expert Strategist: Magatsu's very cunning where he immediately recognizes any advantage he so has during a battle to further achieve his victory. This is shown when he specifically possessed Kohaku in order to take advantage of Sango's familial ties with him and possessed Inuyasha while he was nearby Rin in order to take advantage of Sesshomaru's connection to her. Finally he had Inuyasha send Kagome falling down to her death so that he may possess her when Inuyasha jumps to rescue her. However Magatsuhi's sadistic arrogance was his undoing as he underestimated the strength of Inuyasha's willpower and demonic powers that led to his destruction by Sesshomaru's Tenseiga.

Immense Demonic Power: Representing the evil of the Shikon Jewel, Magatsuhi is extremely powerful, having backed both Inuyasha and Sesshōmaru in demon form into a corner. His power appears to exceed even Naraku's, calling himself a league far beyond his. Shape-Shifting: Magatsuhi has the ability to transform a part or his entire body into a plethora of large, thick tentacles capable of deadly constriction as well as used for impaling enemies. Magatsuhi has no definite form. Like Naraku, he, too, is an amalgamation of demons, briefly borrowing flesh from Naraku to assume a physical form in the material world.
 * Immense Corruption: The darkness of Magatsuhi's demonic powers is so vast and dense that a simple stare corrupted and temporarily knocked out Kagome and simultaneously sealed away her spiritual powers. In fact he even managed to corrupt the Shikon Jewel shard within Kohaku's neck with a mere touch while Naraku can't even touch it without risk getting purified by Kikyo's light imbued within it.

Immortality: As Magatsuhi is the demon spirit residing within the Shikon Jewel, he is virtually immune to all forms of physical damage as since the form he was in during his initial battle against Sesshomaru is borrowed flesh from Naraku, he was able to survive a decapitation like nothing. This also extends to his spirit form where being a mere spirit left him unable to be harmed by physical attacks and only Sesshomaru's Tenseiga was able to vanquish him.

Possession: Magatsuhi can possess another's body. He did this to Rin, Inuyasha, Kohaku and narrowly done so to Kagome as well. However his possession is not absolute as his spirit can be drawn out of another's body by a strong enough Yoki such as Inuyasha's Yokai Form's Yoketsu. Miasma: Magatsuhi's miasma is one of the strongest if not the strongest in the entire series. His miasma is strong enough to seriously injure Sesshōmaru who is very much immune to poison and is of course fatal to humans. His miasma almost kills the monk Miroku who, if not for Magatsuhi fleeing from Sesshōmaru through Miroku, would have certainly died.
 * Weapon Possession: During his possession of Kohaku, Magatsuhi was able to possess the demon bones used to create the demon slayer's kusarigama to cause it grow and extend itself to attack at unsuspecting angles.

Regeneration: Magatsuhi's true form is able to regenerate within a matter of seconds, or become the several demons that he used to be, as demonstrated when Inuyasha attacks him with the Wind Scar in the final battle to save Kagome.