Magatsuhi

Magatsuhi is the 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.

History
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 series, 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. Magatushi 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 killed by Sesshōmaru.

Magatsuhi's spirit is combating Midoriko inside the Sacred Jewel. However, he is split into the many demons that formed him. Magatsuhi 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.

Inuyasha refuses to believe Magatsuhi, and proceeds to attack it 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, and Inuyasha and Kagome are transported back to earthen ground.