This is originally a melded Shadowrun/Earthdawn setting. This is generally not significant to the usefulness of this setting in other genres.
The Caern lies beneath a very large city, with only one further condition to the city: An earthquake has occured here within or near the city. To reach the Caern, one must descend through a cleft opened in the ground by that same earthquake - it is likely that the adventurers will be requested to descend by an ancient being such as a dragon, whom cannot descend himself. Upon descending that cleft, which is sheer, slick, and wet, and most certainly requires climbing equipment. the PCs will descend into a short, broken tunnel. Upon reaching the end of the tunnel, they will find a flat wall. In the presence of a magic-user, a complex runic pattern will glow with a faint reddish light, and the PCs have reached their first puzzle.
The runic pattern is incomplete - Any character with significant magical ability, no matter how it is expressed, is physically able to complete the pattern, so long as he is not marked or controlled in anyway by any demonic / Horrific forces. The puzzle may be drawn out for effect, or reduced to simple rolls.
Once the lock-puzzle is opened, the door will slide aside with a slow, grinding sound, revealing a dark, unlit entryway. A dungy, reeking half steam, half smog will roll out of the door, coating the adventurers in a thin, dirty feeling slime - A stairway leads down into the darkness, where an ancient city slumbers.
This city is ancient beyond human belief: Entire civilizations have risen and fallen outside, and the architecture should reflect that age: Pre-Mayan / Egyptian sort of work. Much of it is now in the process of crumbling, and a half inch of reeking warm sewage fills the streets. Four kinds of beings patrol these streets.
First, and least in number, the sewage spirit. A toxic spirit born of human wastes and the suffering of those it affects, these creatures manifest roughly humanoid bodies of urine, feces, and industrial wastes. They are slow and not terribly strong, but their primary weapons are the diseases and poisons they carry within.
More common are the wandering fire and water elementals that patrol the streets, acting as semi-intelligent servants and pets to the fourth race here.
Finally, the steamlings are masters of this realm, or at least once were. Descended from humans with powerful elemental masteries, these creatures are half man, half elemental spirit. Though they have the body of a man, the steam that is their birthright creeps through their skin, the source of the dense, hot fog here. However, they are desperate, and dying from an entire cornicopia of diseases, and the party likely has exactly what they need to survive: Clean fuel, clean food, and clean water, and the equipment to leave the Caern, which is non existant down here. Steamlings, however, speak no modern language, except the edge of a blade, and they’re so -very- hungry… Their combat talents are varied, though a small number of them will be exceptional fighters, preferring 2 short blades, and a nearly equal number will be archers of some skill.
Buildings may be populated nearly at will by the DM, with one caveat: Any writing is of a form only the most curious of paleo-linguists would even be familiar with, and even then, very difficult to read. Suggested places of interest include training halls for archery, swordsmanship, and magic, the smithies, what remains of the sodden farms and diseased food stores, and the temples, which are raised to deities the party may not recognize, but particularly to a god of passion and a god of guardianship.
Should the party work its way through to the center of the city, there will be a central temple, one upon which the sewage rains from above, seeping through an unrepaired crack in the city systems far above. Inside the temple is a pool of polluted water, and a great, smoking, bonfire which burns with no appreciable source. These are guarded by two creatures, who, like their offspring, may once have been human.
The first is a man, who exudes a smokey flame of the same color and consistancy of the bonfire. He is an archer of magnficent skill and power, and wields a bow of wind and missiles of pure fire.
The second is a woman, whose once fair, but now pox-ridden skin weeps tainted water, is a warrior, and she carries twin blades made of twisting, shifting water with an impossibly sharp edge.
These two will make a bone fide attempt to communicate with the party, and though they will defend themselves, will attempt at least once more if attacked. They desire the party to take bits of the flame and the pool to the surface, and to purify them, with an unspecified reward indicated. This will allow them, but not their newly leprous progeny, escape into the outside world.
Okay, that’s the weakness. It’s hard to get a party down here, short of someone who knows about it saying ‘go fetch me xxxx from the city down there.’ Alternatively, they may stumble upon ancient references in a wizard or historian’s tome, and choose to chase it down. As a last ditch, there’s always ‘Why is that floor steaming?’