The Demonland supplement uses the basic Dungeon Crawl Classics rules, with an alternate XP and level progression, to allow Dungeon Crawl Classics to be easily used with (an alternative version ?) of Tékumel: The World of the Petal Throne. I've never been invested in Tékumel, so this is not a product that I can easily comment on.
The Demonland supplement was mentioned on Spellburn here.
This post in People Them With Monsters discusses the supplement.
Scattered over Tekumel are innumerable half-buried, half-forgotten ruins. There are fragments dating back to the prehuman ages, when the Ssu and the Hlyss vied with one another for control; there are tunnels of melted rock and steel constructed during the days of man's first glory; there are jumbled heaps destroyed by the cataclysms which rent Tekumel when the planet was cast into outer dimensional darkness; there are catacombs and subterranean labyrinths dating from more recent empires, cities, temples, pyramids, and fortresses dedicated to the lost and unremembered gods of half a hundred kingdoms.
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