The Cauldron

The Cauldron is a vast, subterranean superstructure largely located underneath the city of Highspire. Its size encompasses a roughly spherical region beneath the city proper that goes at least as deep beneath the ground as the mega-skyscraper of Highspire rises into the air. Whether it was originally part of or connected to that structure is unknown, but what is certain is that the entire city uses it for a very different purpose than it was probably intended: as a dump.
Understanding Cauldron requires one to understand Highspire. The enormous central tower of Highspire contains the vast majority of the city's inhabitants, and they very consciously divide themselves by economic class, with higher portions of the tower requiring increasing levels of wealth. That means the citizens of Highspire literally look down on people who are poorer than they are, and many of the city's idioms and traditions evidence a fascination with height. It's perhaps unsurprising then, that residents of the upper levels are inclined to ignore trouble that is beneath them.
Highspire epitomizes the saying "slime flows downhill," and the existence of Cauldron is a dark reflection of the much hailed scientific and cultural progress that has made Highspire famous. Throughout the structure, numerous passages and conduits run through building both within and between the numerous levels. The inhabitants use some of them as garbage chutes, whether or not they're supposed to, and the vast distance that trash falls deep below the ground level of the building was for many years so far out of sight and mind that few people thought twice about the practice.
Highspire is justly famous for innovation in technology, magic and alchemy, and produces an enormous amount of industry, with a correspondingly enormous amount of waste. As the vast quantities of materials began to pile up beneath the city, Cauldron began to develop its own ecosystem. Failed experiments mingled with escaped lab animals, and local wildlife unlucky enough to stumble into an open shaft near the city encountered bizarre chemicals and the residue of half-baked enchantments. Some of the creatures released below were even designed to be hostile. Breeding monsters and beasts for use by armies and mercenaries is a profitable business in Highspire, and the numerous regulations that are meant to keep the citizens safe from such creatures mean that more than one rogue alchemist with the city watch at his door has consigned his experiments down the nearest chute to the Cauldron rather than be caught red-handed with illegal mutation subjects.
A small and very unlucky group of maintenance workers were tasked with cleaning out the ducts and ensuring that everything continued to flow smoothly, and for a time this was done without incident. About 50 years ago, however, the number and ferocity of the inhabitants of the underground megastructure began to reach a high enough concentration that Cauldron started to become dangerous.
At first the increasing disappearance of maintenance personnel was explained away. It was a miserable job, who wouldn't quit and strike out on their own? There are any number of ways to take a bad fall down the shafts, and who knows what noxious fumes they may have tragically been exposed to as they swept up the city's collective mess?
The problem steadily worsened, however, and soon there were reported instances of attacks in the very lowest levels and in the small villages that surrounded the spire, which sit over numerous vents and tunnels connected to Cauldron. As the problem began to grow out of control, those most responsible for the problem wanted the least to do with it, and mages, chemists, alchemists and inventors used their considerable social and economic influence at the top of Highspire to ensure that they retained the ability to dispose of their trash with convenience, and that the disturbances were treated as isolated incidents. Their propaganda and assurances were sufficiently reassuring that when the first wave of "The Tide" arrived, it caught everyone unprepared.
For reasons unknown, ten years prior to the start of the campaign a vast horde of monsters, mutants and strange clockwork nightmares poured out of Cauldron into the lower levels of Highspire and across the surrounding region in an event now called "The Rising of the Tide."  Thousands of innocent people were slaughtered before anyone realized what was happening, and such was the ferocity and suddenness of the attack that the city advised those living near the walls to evacuate entirely and pulled many of the inhabitants of the lower levels into the upper tower until they could organize a reaction.
Despite the frightening and surprising nature of the assault, after the initial shock wore off the leaders of the city weren't too worried. Highspire had a significant standing army outfitted with some of the most advanced magic and technology on the continent. Surely they could beat back a few overgrown lab rats?
In fact they could, but only just. For over a week an offensive raged down the lower levels of the tower, with the army sometimes punching through to Cauldron proper, and other times being forced back to the upper levels. The creatures were not only ferocious, they seemed to be coordinated in a way that defied common explanation. A diverse array of monstrosities behaving in perfect concert, the military quickly realized that this was less a stampede than an invasion. After significant losses, and much destruction across the lower levels, the creatures were driven back below and most of the entrances to Cauldron were sealed.
The city declared the problem solved, and set about trying to go back to normal. They discovered that large quantities of the raw materials that were refined, smelted or assembled on the lower levels had been taken by persons or creatures unknown, and that vicious traps had been laid to cover the horde's retreat. To this day, every few weeks or so there's a story about some unfortunate in the lower levels who triggered a leftover trap from the Tide. It happens often enough that rumors have begun to spread that someone is still setting them. Are some of the creatures still hidden in the lower city, wreaking their vengeance from the shadows?
In any case, there was much more to lose than gain by attempting a full invasion of Cauldron. The military had lost many soldiers reclaiming the lower levels, and rising tensions with other nations necessitated their presence elsewhere. A watch was set on this new border to "the city below," and laws were passed that strictly punished those who disposed of dangerous materials improperly. At least, those that got caught. These regulations accelerated an already troubling migration of renegade scientists and mages establishing their own secret laboratories in Cauldron away from prying eyes, scavenging among the junk for materials and relying on their own guardbeasts to keep safe a few private rooms where they could trade with smugglers and conduct experiments far from the eyes of the law. The recent fighting had left plenty of corpses for research and the undercity's inhabitants had been sufficiently thinned by the military's campaign that the upper levels were experiencing a power vacuum.
These migrants below began to organize into groups for mutual protection, groups that seemed to combine the worst elements of corporations and street gangs. A kind of no-man's-land developed, with the upper sections of Cauldron becoming the destination of choice for those who wished to disappear or run where they would not be followed. Some even entered the darkness willingly, hoping that the more desperate and chaotic circumstances would prove their chance to make their fortune.
While the official word is that the great structure known as Cauldron has been pacified, and that the groups in the upper levels are merely beggars and outcasts, even the most dishonest of officials can't pretend that Cauldron is empty. This reigned for a time as the new normal, and a black market developed that made troubling inroads with more legitimate business interests further up the city.
However, the fragile peace in Cauldron is coming to an end. Over the last year there has been a sharp increase in disappearances in and around the base of Highspire and its lowest levels. People in the surrounding villages are afraid to go out at night. Reports of strange behavior and movements suggest that it is not so much a matter of if Highspire will once again have to face the nightmare of the Tide, as when. Gangs, criminal organizations and even some monsters are being driven up from the depths in greater and greater numbers as Cauldron once again reaches a boiling point.
In the meantime, the city has grown desperate both to assure everyone that the problem is being handled and to try and forestall what now seems inevitable. Adventuring companies are being hired to sweep lower levels, investigate reports of smugglers or criminals using the upper sections of cauldron, hunt especially violent or dogged monsters and generally try to keep things contained.
The city is also, however, at least trying to be proactive this time. The true cause of the terrible intellect and organization of such a motley and dissimilar army of creatures and constructs was never discovered, and some city officials believe that locating whatever terrible intelligence first organized the inhabitants of Cauldron to quite literally rise up is the key to keeping history from repeating itself. Is there such an intelligence, and has it once again set its sights on the surface world? What role will the new human populations of Cauldron play when this dark army reaches their doorstep? One thing is certain: the Tide is rising again, and this time it may be too powerful for anyone to stop it.

Purpose / Function

The original purpose of Cauldron is a mystery as well. Why is it underground? When was it built, and by whom? And why did the people who settled the city settle in the upper sections in such numbers but initially neglect to house any significant number of people underground (before it became a trash dump, that is)?

Alterations

There are essentially three "eras" of Cauldron. First came the construction of the facility itself. How this was done and why is not entirely clear, but at least some of the space seems intended to house a large population.
Next came its time as a dump. Maintenance workers installed sluices, converted hallways to waterways, lay piping and generally tried to make it as easy as possible for garbage to get as far down as possible with as little work on their part as they could manage. Reinforced sections, chemical containment and treatment areas and even a limited laboratory and forward operations center were created.
Finally, the Tide and its aftermath. When the Highspire army returned to some of the lower levels, they found significant changes. Some areas had been converted into workshops or factories to produce more of the horrors that would attack the city. Some sections had been modified to house or breed various creatures. Some sections were just plain torn up from the large angry monsters that had lived there in a confined space for so long.
Much of that damage and construction was appropriated by the guilds and gangs that subsequently populated the upper levels. Living areas, shanty towns and a black market were established. Several of the factions sealed off and modified parts of Cauldron to serve as guild houses and bases of operations. Illegal laboratories flourished, and some creators even managed to get relatively small sections of factories working, assembling various devices or clockwork creatures. Naturally, wandering monsters and chemical hazards are still a huge issue, and only a fool would expect to journey through any significant portion of even the most inhabited areas without a means of handling trouble. The Watch does not enter Cauldron as a matter of policy, and what protection is available is expensive.
Meanwhile, deep below all of that but increasingly closer to the surface

Type

Acropolis / Citadel

Parent Location

Highspire, aka The Towering City