Gadzooks, this is The Gates of Firestorm Peak (1996) and what a mess!
It’s not Bruce Cordell’s fault, really. This scenario was commissioned as a showcase for the 2.5E Player’s Option Rules (Combat & Tactics, Skills & Powers and Spells & Magic) and it’s a compelling argument to never use them. Amusingly, the villain is a wizard corrupted by outré forces, expressed by the fact that his spellcasting uses magic points. I don’t think that’s intended as commentary, but I’m taking it as such anyway. Thankfully, all this stuff is optional and can be completely ignored. It takes up a ton of space, though.
Without all that bunk, you’ve got a pretty solid, lengthy and icky dungeon crawl. The idea is that the titular gates only open once a generation when a particular comet is in the sky. The father of one of the player characters went in 27 years ago and never came out, so, well, go and get him. The outer reaches are a pretty standard, if very dangerous, slice of the Underdark populated primarily by duergar. Deeper, the party finds horrors from the Far Realm have seeped in. In fact, this is the first published appearance of the Far Realm and some of its nasty, Lovecraftian inhabitants. It would go on to become a key locale in later D&D editions, and particularly for the Eberron setting. Aside from extradimensional monsters, there’s some more conventional horrible stuff in dungeon as well. I particularly dislike [complimentary] the fountain, which Arnie Swekel illustrated just to drive home how disturbing it is.
Swekel was a good choice of artist for this, he is like a latter-day Russ Nicholson who excels at illustrating player character types in their death throes without making it too disturbing. The Jeff Easley cover ties the action to the Player’s Option books, but feels somewhat out of step for the more horror-oriented tone inside.
In a weird bit of packaging design, the adventure book came packaged in an identically fronted file folder, which contained counters and a battle map as well. My second-hand copy was missing those, but the folder…is unusual! A couple of the later Planescape modules had a similar folder, but I don’t know of any TSR products that were configured quite like this.
