The Great Pentagram rests in the middle of Daemon's Crag in Ultima VIII: Pagan. It acts as the prison of Pyros, and whenever he is called, he will appear at this place, since it was here where he was first bound to the Tongue of Flames. However, should said artifact touch the middle of the Pentagram, Pyros will be freed, to destroy the world in his anger over his imprisonment.
The Avatar was a participant in one of these ceremonies, during which Malchir called Pyros to ask him about the sun. Later however, the hero returned and put the Tongue of Flames into the center, effectively freeing Pyros to run rampant.