Onigumo

A mere bandit who suffered severe burns and a fall off a cliff. He was found by the priestess Kikyo and taken to a cave where she bandaged him and tried to heal his wounds. But Onigumo would never walk again, and suffered a severe burn mark on his back shaped like a spider.

Over time, Onigumo began to both desire Kikyo, and covet the Jewel of Four Souls. Kaede grew nervous and mistrustful of the wicked Onigumo, but Kikyo urged her to take pity.

Onigumo's evil would not be contained, and as Kikyo's powers weakened demons began to appear and Onigumo offered these demons his body and soul to devour. As the bandit was devoured, a new and more terrfying evil began to take form. It was the birth of the demon Naraku.

(While Naraku now exists in Onigumo's place, it has recently become apparent that Onigumo's heart still beats within Naraku.)