InuYasha (2000)

S1E26 The Secret of the Sacred Jewel Revealed

Sango shares the hidden tale of how the Shikon Jewel came to be. Midoriko, a priestess with immense spiritual strength, fought fiercely against numerous demons in a life-or-death struggle. During this battle, she absorbed the demons' souls, ultimately sacrificing her own in the process. When her soul was expelled, it transformed into the Shikon Jewel.