To enter the illusion event you will require an Illusion Pergamin that acts like an invitation to the event. In order to obtain this item you must combine a Potion of Illusion and an Scroll of Illusion(of the same level), along with one Jewel of Chaos at the Chaos Goblin in Noria:
Note: The combination can fail resulting in the loss of all materials.
You can right click the Illusion Pergamin to find out how much time there is left until the start of the event.
Once the event start has been announced you have 5 minutes time to go to Mirage the Mummy(lorencia upper vault) and register for the level range you are in using the appropriate Pergamin: