Absolute Anime

Midoriko was a priestess [miko], much like Kikyo. She used spiritual powers not only to subdue demons, but to purify them as well.

Then came the battle. She dressed herself in miko robes and old armor, and wielding only a single sword, she began battling countless demons. The battle raged on for 7 days and nights, until finally the many demons she fought formed into one.

When this demon got Midoriko in it's jaws, Midoriko used the rest of her powers to force something out of her chest. That something was the Shikon Jewel [Shikon no Tama], or jewel of four souls. She imprisioned herself inside the jewel, along with the demon. The battle still goes on inside the jewel.