When I first launched my Caverns of Thracia campaign I was actually planning for nothing more than an experimental one-shot using the original 1974 rules for Dungeons & Dragons. The framing device I chose was relatively straight-forward: The Caverns of Thracia are located beneath a cluster of surface ruins in the midst of a vine-encrusted jungle, so I simply based the PCs out of a small logging village near the edge of the jungle. I arbitrarily decided that the ruins would be located 1d6+2 days of travel into the dungeon and used a 1d8 roll to randomly determine the compass direction from which they would approach the ruins.
As the would-be one-shot developed into an open table campaign, however, additional details began to accumulated. The village has grown into a dungeon-fueled gold rush town and, in the process, it’s been considerably fleshed out by the players: For example, one fellow had the memorable back-story of being the chef at the local tavern. (He dressed up in a suit of plate armor and pretended to be a well-trained knight in order to sign-up with one of the adventuring parties heading to the caverns.) Another PC, a halfling named Himbob Jimblejack, retired from adventuring and used the wealth he’d gained to buy up all the local garlic farms and corner the market on garlic. (HIs player is quite hopeful that my recent interest in Ravenloft will spike the local market for garlic.)
Meanwhile, the once nameless woodland has become the Intemperate Jungle, so named because it has no business being a jungle at all. The land all about this particular jungle is temperate in clime — roughly equivalent to western Europe. But the jungle is, nonetheless, sultry, moist, and unnaturally verdant. (The logging village prospers, in part, because the trees of the jungle regrow at a preternatural rate.)
Rumors abound that the jungle exists due to the same terrible curse that destroyed the Empire of Thracia. But since that curse is, itself, nothing more than a rumor, the truth may be something else entirely.
(From a metagame perspective, the Intemperate Jungle exists because I wanted to place some other old school classics in near-proximity. The Barrier Peaks are now located just north of the Intemperate Jungle, and the Palace of the Silver Princess is actually nestled into Mt. Karnath at the eastern end of those peaks.)
A unique aspect of the Intemperate Jungle are the pollen monsoons. The far western edge of the jungle, where it grows to meet the sea, is filled with massive, flowering trees. When the season is right and the hot sea winds blow in from the coast, massive clouds of pollen are swept east across the jungle and out across the plains beyond. At those times, the logging village is forced to shutter its doors and windows: Visibility is reduced to almost nothing and the thick, cloying pollen can choke a man to death.
(Metagame Perspective: In mid-2010 there was a lengthy hiatus from our Caverns of Thracia games because the Complete Readings of William Shakespeare combined with some other theater projects to mean that virtually every single evening was booked full. When we returned to the game, I made an impromptu decision as the session started that there should be a gap in the chronology of the game world, as well. The idea of a pollen monsoon popped into my head.)
In the aftermath of the pollen monsoon, PCs adventuring deep into the jungle discovered a new danger: Pollen cysts. These pockets of pollen had accumulated in various nooks and grottoes of the jungle, and when disturbed they would send up miasmic clouds of choking pollen into the air.
(Metagame Perspective: Pollen cysts were created during our last session as the result of a wandering monster check during their journey to the caverns.)