From the athas.org community website:
Far to the south of the cities of the Tyr Region, beyond the ken of common men, there lies a void, a vacant wasteland of purest black. There is no life there, none at all, but there is a yearning and a pounding of withered hearts in the husks of truly ancient undead. Vibrant civilizations of t’liz and zombies, meorty and skeletons, throwbacks to a cataclysm unleashed during the Cleansing Wars that scarred the face of Athas with a vast plain of elemental obsidian.
All that saves the city states from their wrath and hatred is that they are convinced their world ends at the edge of the black glass. Woe to the living if they ever find out the truth.
Secrets of the Dead Lands explores this vast obsidian plain and the undead civilizations that thrive there. Their rulers, ancient dead despots and champions, viziers and duchesses, vie for power and territory, animating, destroying, then reanimating the pieces of their armies in endless struggle. From a time before the Cleansing Wars had rid the world of so many races, there are undead trolls, gnomes, pixies, and more, creatures with no living counterparts anywhere on Athas. The humanoids gather for only one apparent purpose: to keep the unpredictable undead insects, the “bugdead”, from swarming the world.
Whoever said the most terrifying fables can’t be true?