Onigumo

was a bandit who was rescued by the priestess named Kikyō. He fed his body to yōkai and ultimately became Naraku.

Origins
Onigumo was an evil bandit who gained his injuries in a fire that was caused by Rasetsu who set the building that he was in a blaze because Onigumo had betrayed him and taken his men. Rasetsu then had his men throw Onigumo off a cliff to finish him but he managed to survive.

Kikyō found him, nursed his injuries in a cave, and occasionally, Kikyō's younger sister, Kaede took care of him as well, even though she didn't like doing it. Onigumo originally had difficulties accepting Kikyō's kindness, and told her to forget about the bandit when she visited him, but Kikyō's kindness had also made Onigumo fall for her. Onigumo gave up his body to demons in order to regain his strength, mobility, and power.

Onigumo did this because he desired Kikyō and the Sacred Jewel for himself, while the demons did it because they wished to get rid of Kikyō and obtain the Sacred Jewel. This created the evil half-demon, Naraku who then immediately tricked Inuyasha and Kikyō into becoming enemies so he could have the sacred jewel for even more power than he already had. His deceit led to Kikyō's death and Inuyasha's sealing to the Tree of Ages.

Reborn
When Naraku removed his human heart, he also removed Onigumo's soul in the process, which became Musō. Much later, Musō finally remembered about his past as a bandit and of his time with Kikyō within the cave. Muso was eventually absorbed by Naraku, becoming part of him again.

Final Death
In one of Naraku's final moments, after Inuyasha destroyed forever his body with Meidō Zangetsuha, he recalled that Kagome had said the Jewel did not grant his real wish. Naraku recalled that what he really wished for was Kikyō's love, but it seemed that it was not granted to him. Before Kagome's arrow had pierced the jewel, Naraku's soul commented that he wouldn't be able to go where Kikyō had gone. It was presumed that Onigumo's emotions had somehow surfaced within Naraku as he recalled his true wish, thus proving what Kikyō had once stated to Naraku was indeed true: "You've disguised yourself well, but you're still that fugitive human, and who you once were can never be erased".

Naraku's soul was sealed inside the Shikon Jewel after his purification from the material world. In the manga, Naraku's soul was destroyed forever by Inuyasha when he cuts the point of light in the darkness inside the Shikon Jewel with Meidō Zangetsuha, which is the key to kill Naraku and destroy the jewelry evil. In fact, with Inuyasha's attack at this very point, the purity of the Jewel called Naohi annihilates the evil corrupting presence within it, then the Jewel's shimmering and voice completely stopped.

Instead, in the anime version, when Kagome wished for the Jewel to be destroyed, Naraku's soul was purified and went on to the afterlife. The Onigumo part of his soul seemed dominant at this point, and was finally at peace, where the demon half of Naraku's soul presumably separated from Onigumo to go to hell.

Trivia

 * Onigumo had a spider mark on his back, even before he became Naraku.
 * Onigumo (鬼蜘蛛) can also be translated as the, a particular species of orb-weaving spider.
 * It was Naraku's actions that brought some sense of irony with Onigumo's words of how he would have liked to see Kikyō "apprehensive and frightened," a facial expression that was made as Naraku struck down Kikyō, as he was disguised as Inuyasha, though Musō, Onigumo's later reincarnated form, expressed a large amount of anger and regret as he recalled that bitter memory as Kikyō had died from such a mortal wound due to Naraku's miscalculation.
 * As Musō, while Onigumo did remember about the Shikon Jewel, he apparently did not seem to desire the jewel as much as he did with Kikyō, which implied that Onigumo saw the Shikon Jewel as merely a bonus and his main objective was Kikyō.
 * The way Onigumo's irises are drawn makes them look like they contain a dark spider web.