WG4: The Forgotten Temple of Tharizdun
The Forgotten Temple of Tharizdun is a Gary Gygax adventure module for Advanced Dungeons & Dragons. It is WG4 to Lost Caverns of Tsojcanth's WG3 (published as S4).
The cover is famously eye-wateringly bad, although one could argue - like Gates of Firestorm Peak - that it serves not to tip off nosy players into the horrors within. Ah whom are we fooling, the cover is utter crap.
The temple is on a Teotihuacan-like box-on-a-pyramid plan. The inverted two-step pyramid is a symbol of this particular Tharizdun cult, which Monte Cook reused in the Temple of Elemental Evil remake/sequel. Tharizdun later developed into the Azathoth/Cthulhu figure for the Greyhawk setting.
Here is a subversion of dungeon-crawl tropes: as the party gets closer to the heart of darkness, they find more and more evidence that they Should Not Be Here.
Where Tsojcanth debuted creatures as would (soon) enter the Monster Manual II; the Temple reaches back to the Fiend Folio for several of its monsters.