Magatsuhi is the youkai 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 youkai; however, unlike Naraku, who retains a humanoid form, Magatsuhi's true form is much more monstrous. 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 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 Shikon Jewel.
Magatsuhi states that if Kagome were to make a selfish wish on the Jewel, Kagome would replace Midoriko in the eternal fight and Naraku would replace Magatsuhi. InuYasha refuses to believe Magatsuhi, and proceeds to attack it in an attempt to reach Kagome. 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 Meidou Zangetsuha and arrives in the meidou outside the jewel to save Kagome. Then Kagome makes the right wish; the wish that would exterminate the jewel from the world forever; the wish for the Shikon no Tama to disappear. The jewel breaks; disappears, and InuYasha and Kagome are transported back to earthen ground.