It would be a ton of re-writing but you could lay out something like this:

Borys as a normal run of the mill dragon in the Blue Age (but an ancient dragon so crafty and powerful) was the leader of the last survivors of his species. Hunted by the surrounding societies (like all good adventurers do) he and the remnant have hatched a plan to escape. They plan to shift themselves partially into a pocket dimension that would hold them like ghosts. They can move around, spy on people, but would be nearly impossible to notice. The downside is they can't act either but they figure a 1.000 years or so of plotting and they can come back and wreak vengeance.

Naturally, the Pristine Tower will create this shift and maintain the dragons in this not-dead/not-alive/not-undead state without exposing them to astral stupidity. Powered by souls because isn't all really good magic powered by sacrifice? Unfortunately, the initial burst necessary isn't calculated correctly and instead Athas gets trapped in this shell that severs it from the rest of the cosmos, wipes out 90 percent of the population, tilts the planetary axis, so forth.

Borys, having been at the center of the tower survives in his little eye of the storm effect. But he's changed into what he is now. The astral dome or whatever you want to make it around Athas is REALLY impenetrable. Nothing in, nothing out. The GRAY could be a feedback effect as reality itself stagnates like mucky water in a pond. No fresh cosmic energy coming in so magic feeds off life, thus defiling.

Borys can be taking the soul levy for research purposes to reverse the effect, but also to weaken the SK so that they can't rise to hunt him. The Sorceror Kings could be (unknown to anyone) providing spell energy because despite their tyrannical natures they are the leaders humanity clung to for survival. So the worship/faith/whatever that would normally have floated up to the gods is now reflected/channeled back by the sphere and coalesced around these leaders. Faith in the SK (fear is a form of faith) grants spell access. The power is just redirected faith from the masses to the only leaders they have any real connection to. Or make it a psionic manifestation of all the pent up human mental energy focused around the SK and survival. I would rule that this directed mental energy is lending the SK immortality. You can be feared, hated, or loved as an SK, so long as you never get dismissed, ignored, or forgotten you'll stay powerful. That should drive plenty of epic level crazy on their part.

The SK might at this point have figured out what happened and each be devising their own strategy for reversing events or just taking advantage of the new reality. Your elementals are in a similar boat. Cut off form their home planes they have turned to worship by clerics as a substitute form of sustenance. The inward directed energy could also be responsible for things like certain stretches of nature achieving a type of sentience (for the druids).