Unfortunately, if you left the dungeon, you lost all of your progress, and you will have to start at the beginning again.
If this doesn't help, you might want to send an e-mail to firstname.lastname@example.org and explain the problem to them. Be sure to give them your UserName in the game, and the name of the wizard who is having this problem, so they can help you faster. Maybe they can get you moving forward with this quest.
Thank you for the reply. i appreciate your help very much.
Indeed, i have completed all necessary quest for the plague oni to appear. I did leave the dungeon and will be more than happy to repeat it, however, i will be quite unhappy to repeat this over again for him not to appear once more.
I will follow your advice and contact support if i have this issue again. Thank you once again.
I also am having the same problem. I have done everything that needs to be done before the plague Oni quest but after defeating everything there the Plague Oni did not appear. Just not sure what to do. I wrote a bug report to wizard101 a week ago but still have not heard back. I'm at a loss for what to do. I cant move on in my quests without passing this one.