Very cool.
Actually, I was going to leave Kyuss as a full-blown god, who survived their extinction by virtue of being imprisoned. Of course, that leaves the PCs with a dilemma - is it better to leave the world a godless wasteland, or hand it over to a god who is himself awful.
However, where I was running into more trouble was the giant/dragon conflict in "Kings of the Rift", and also what to do with Dragotha.
Nice idea with Kyuss. I am not as familiar with Athas, but maybe Dragotha is an ancient sorcerer king who long ago was given lichdom by Kyuss and laid waste to a land far beyond the had cliffs. The conflict with the giants happens in the jagged cliffs. Dragotha is so powerful that he conscripted other sorcerer Kings over the ages to his service. The heroes then teleport in adventure 11 to the rift which is far beyond the jagged cliffs.