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How do you use this fascinating location in your AS&SH campaigns? As a child I read tantalizing hints about the Plateau of Leng from Lovecraft and was later surprised to see other writers like Stephen King and Robert Anton Wilson mention it in their works. The "canon" information is surprisingly limited and its location unclear (it seems to simultaneously exist in several locations like the Central Asia, Antartica, the Dreamlands, and in my canon, Hyperborea). I was excited to see AS&SH include this location and am very interested to see how it is used in other campaigns. At the time of writing I see it as a sort of negadungeon- that is, there is no reason anyone in their right mind would go there. The Plateau of Leng has no fabulous treasure to rescue or beautiful princesses to save, all it offers is death, madness, misery, and Things Man Was Not Meant To Know. That said, my campaign is still in the idea phase and I would enjoy hearing other ideas.
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"things man was not ment to know" sounds like sweet irresistable esoteric lore and spells to a party of magic users. bard's in particular (see satyr/man of leng beastiary page 67) to study the mad pipings of azathoth's keepers and what not (also see sharath, pg 214 hyperboria gazeteer - spacifically paragraph 5).