What I gathered:
Tharizdun and Entropy are trapped on Oerth Prime. In this sealed off world the two powers will battle each other for a millennia. Eventually, Tharizdun will expire and Entropy will be let loose upon the cosmos once again. Gord and his friends are given life on an alternate world named Yarth.
This was made possible due to the workings of Proctor Chronos and Lady Tolerance.
At the end of the novel, Gord, Gellor and Leda jump from plane to plane. Tharizdun is in pursuit of them and begins sealing off all portals through which energy and assistance can aid the three heroes. Eventually, Gord and company arrive under Castle Greyhawk on Oerth. This is the very center of Balance; Lord Yang and Lady Ying reside here. Gord believes that spheres beyond the reach of Tharizdun can be accessed at this place.
This is exactly what both Tharizdun and Entropy want, and in fact, have lead them here. They believe that the source of the heroes power (Courflamme and the three bands created by the Theorparts) are at their weakest beneath Castle Greyhawk and can be destroyed, ending the conflict.
But all of this has been planned by Proctor Chronos and Lady tolerance. This site, once a portal to the multiverse, has now been sealed off. When Tharizdun destroys Courflamme its three components give life back to Gord, Gellor and Leda, and when Entropy destroys the three bands, they and many of Oerth's inhabitants are sent to the alternate reality of Yarth.
Oerth was destroyed as Gary Gygax was fired from TSR. Yarth is his new beginning.