Once you have gone through the Tree of Life Dungeon and completed it by healing the Tree of Life and defeating the Death Oni, you get the quest to visit a monk outside the Jade Palace. He then sends you to heal the ailing Emperor. Once you go next to the Emperor, he becomes the Jade Oni and you must fight him to free the Emperor. He is a 7500 health Life Oni. I don't remember what he drops though.