After walking out of the Main Hall you see Hokas Tokas, the wizard. He says to you, "I have just heard word of a new tyrant. He has the ancient Medallion of the Mind. It was stolen from the king's treasure room and has the power of controlling other people's minds. I found the tyrant's hide-out: it is in the Chaosium Caves. You must venture into the caves and kill the tyrant. His name is Astrallion."

He makes a gesture with his hand and you find yourself in the caves.

Details at Eamon Wiki