Magatsuhi

Magatsuhi is the yōkai who killed Midoriko and whose soul with whom she continuously battles inside the jewel. Like Naraku, Magatsuhi was created by a corrupt human who lusted after a priestess and allowed his body to be consumed by yōkai. However, unlike Naraku, who retains a humanoid form, Magatsuhi's true form is much more monstrous.

Summary
Magatsuhi is first seen in manga chapter 89 as the mummified corpse of an unnamed but massive demon. Later in the manga, the demon's name is revealed to be Magatsuhi, and it is able to (temporarily) escape with the jewel and create a humanoid body out of some of Naraku's spare parts. However, this body was soon destroyed by Sesshomaru and InuYasha. In chapter 555 in the manga, InuYasha saw Magatsuhi combating Midoriko inside the Sacred Jewel.

Magatsuhi states that if Kagome were to make a selfish wish on the 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 Sared Jewel with the Meidou Zangetsuha and arrives in the meidou 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.