Magatsuhi

is the evil inside the Shikon Jewel, born from the spirit of Jewel's demon.

History
Naraku helps him to manifest in physical form so that Magatsuhi can help him to 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 by Bakusaiga in the fight (having lost the flesh which 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 Miroku and Sango 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 spider 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, but Inuyasha stopped him with Dragon-scaled Tessaiga and is destroyed forever by Sesshōmaru with Tenseiga.

His death breaks the seal on Kagome's spiritual power.

Physical description
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. He resembles to Jūrōmaru and Kagerōmaru. 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.
 * 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.
 * 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.
 * Immortality: As Magatsuhi is born from 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. The presence of a corrupted jewel shard enhances his immortality to the point that he can survive being sucked into Miroku's Wind Tunnel, something previously unseen in any other invincible enemy such as Kaguya or Hakudoshi.
 * 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.
 * Demon Bone 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.
 * Enhanced 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 killed Miroku who, if not for Magatsuhi fleeing from Sesshōmaru through Miroku, would have certainly died.
 * High-Speed 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.

Trivia

 * His seiyuu as unified consciousness, Takeshi Kusao, also voiced Bankotsu.
 * If the Jewel of Four Souls is tainted being held by an evil/greedy human or Yōkai it turns a lurid black-violet, whom is represented by Magatsuhi/Evil within the jewel, means the powerful yōkai is winning over Midoriko, Naohi/Good.