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What adventures are there out there (old school) that use Snake People?
Reading Kull at the moment, there must be a lot of adventures that have tapped this. But then perhaps not?
edit: chainsaw adjusted title
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The adventure right in the back of the 2nd edition book has one as the big baddie at the end of the dungeon.
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They're in Forgotten Fane of the Coiled Goddess. And one of the TSR Conan AD&D adventures (with Arnold on the cover) from the 1980s features them.
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Also (highlight to read a significant spoiler):
Snake-men are the secret antagonists in The Anthropophagi of Xambaala.