Over 25 years later, The Forgotten Temple of Tharizdun still creeps me out.
I'm not entirely sure why, but I suspect, even moreso than its quasi-Lovecraftian theme -- a dark, imprisoned god -- it's the artwork that does it for me. The module is entirely illustrated by an artist otherwise unknown to me, Karen Nelson. This level of artistic unity was unusual in TSR modules, which tended to use several artists. Here, though, the consistent look contributes greatly to the feel of the thing. I find the cover image perfect: its unisex, featureless humanoid surrounded by writhing tentacles/serpents/arms being a superb evocation of the kind of "unfocused" uneasiness I feel about the module.
I say "unfocused" because, as I said, I can't quite put my finger on why module WG4 makes me feel so unsettled. Simply reading the text itself, there's really nothing truly disturbing there. There are no images of graphic violence or even of psychological disturbance. Indeed, on many levels, The Forgotten Temple of Tharizdun is merely a workmanlike Gygaxian dungeon crawl. But I think it's probably a mistake to look for any one thing as the source of the module's strange feel. I think what sets it apart is that cumulative effect of many little details.
I've already noted the artwork, which I found to be (largely) atmospheric. The color of the module itself -- a sickly lavender -- is unique among TSR modules of the day. Tharizdun's colors are black and purple, if I recall, and these hues appear throughout the module itself. Now, black as an "evil" color is somewhat cliched, so much so that it doesn't really have any effect upon me anymore. But purple? Purple is a royal color and second only after pink in the hearts of little girls. How could purple be "evil?" Take a look at the cover of WG4 and you can see. It's a sickly shade -- the color of decay, entropy, and insanity.
The module is a quasi-sequel to The Lost Caverns of Tsojcanth, another excellent Gygaxian dungeoncrawl and one with a similar theme: exploring the former haunt of an ancient evil. Whereas "ancient" in the case of Tsojcanth means several centuries before the present, WG4 takes it to mean millennia beforehand. There's a creepiness that comes from thinking about an evil from the time before time. I think that's another part of what makes this module work for me: Tharizdun the imprisoned god may seem like a Lovecraftian concept -- and it is -- but Gygax didn't share HPL's worldview. He was, as I've said elsewhere, a pretty traditional fellow when it came to moral matters and so Tharizdun, while ancient and thus unspeakably evil is nevertheless evil. That is to say, he's genuinely malevolent; Tharizdun actively wishes to bring ruin upon the entirety of the multiverse. He isn't beyond good and evil -- he is Evil.
Then there are the gnomes. No, I don't find gnomes disturbing, but I do find the use of the gnomes as the framing device for the module to contribute to WG4's creepiness. See, Gygaxian gnomes have more in common with garden gnomes than with the post-Dragonlance mess we call "gnomes" nowadays. They're these unassuming woodland guys who hang out with badgers and moles and are renowned for their trickster natures. Take that image and juxtapose it against an ancient temple dedicated to an avatar of Ultimate Evil and you have to admit that it's jarring. Equally jarring are all the Fiend Folio monsters that make their appearance here. Gygax is known to have disliked the FF and I don't blame him; it's a much more uneven work than his own Monster Manual and even its best monsters can only be called "quirky." And yet here they are in WG4, probably the only time, say, norkers ever appeared in my campaign. This gives the whole thing an "otherworldly" feel to me, as if it takes place somewhere other than the typical Gygaxian World of Greyhawk.
I can't say, as some might, that The Forgotten Temple of Tharizdun is a good module for its actual content. I don't dislike it, mind you, but I also don't think it's all that remarkable. What is remarkable, though, is the feelings it still conjures up in me after all these years. It's a very effective mood piece and one I'd love to be able to emulate some day in my own work.