The last quest is "The Key At Last". You get this quest by talking to the Emperor immediately after defeating Jade Oni. You need to talk to the Emperor BEFORE you leave the Palace, log off, or teleport elsewhere, or you may have to defeat the Jade Oni again. The Emperor will send you to see Professor Armbrose.
The quest directly before that is "An Imperial Cure" given to you by Tan Keng. That sends you into the Palace to heal the Emperor, but you have to fight the Jade Oni first.