I have completed enough quests to be able to go to avalon. Though I no longer have a main quest. I've backtracked on the wiki to try to find something I missed. I'm stumped and don't have any relevant quests currently in my quest book. It seems like some sort of glitch. The wizard I'm getting the problem with is Marcus LionHeart
By the time you're able to view this response, you'd probably have figured out your issue by now.
So, instead, I'll leave a little advice in case a similar issue repeats itself.
Quest Finder - Try using the Quest Finder within the last known location you recall getting lost. If you skip sides, you may find yourself having to accept quite a few before finally getting on the ideal path.
Contact Support - You can also try contacting the support team. Usually, they will be able to assist should you get off track. It helps to provide as much info about the issue as you can such as Wizard Name, Level, Description of problem and your Username (not password).
Successfully completed the quest Chasing Shadows requiring you to hence, complete the Mirror Lake Dungeon.
If you've satisfied this, then the Headmaster should have a quest for you.
If, on the other hand, both requirements are met and the headmaster has to tasks for you, I again implore you to exercise the email above and contact KI.
Finally, if only one/none requirement is met, then you still have some "housekeeping" to do.