Ancient Nithia as a Setting: Adding Dark Sun's Defilers to Mystara
by Marc SaindonDark Sun, the "problematic TSR setting" that wotci will probably never re-publish, had a neat ecological/oil crisis approach to magic with its Preservers/Defilers versions of the Wizard, although it was over-simplified in some gaming tables that Evil magic-users are automatically Defilers, even though an Evil person can actually be pragmatic enough to preserve nature for the only keep himself powerful (he can still fertilize the soil with the blood of the innocents and the corpses of sacrifices to maintain his Alignment).
In Mystara, the Nithian civilization turned to Entropic immortals, it angered some people, and the Nithian Empire was wiped out of history. Good enough Nithians got transported into the Hollow World, while on the Isle of Dawn the colony of Thothia was allowed to continue, with some amnesia.
What if there was something more, like Nithians actually managing to unlock the secrets of Defiling magic from Athas, and popularizing it enough that it warranted that the immortals intervened and deleted enough of Nithia from the collective mind to remove this sort of magic from Mystara? But before the immortals act, a group of adventurers face these new Defiler antagonists, trying to prevent Nithia from becoming a desert, setting up a bronze age campaign (in which you can insert more elements of Dark Sun).
What if their ctr+alt+del wasn't that thorough, and some source (Zargon?) can still teach this form of magic? Maybe he already has, and this what Barimoor is secretly working on. Or it might a new Secret Craft in Glantri, because a third's a charm in necromantic Principalties, and they are real keen on keeping Clerics out.
map from: https://www.deviantart.com/6inchnails/art/Ancient-Nithia-500BC-739324630