Plex
Plex is a living world floating in the plane known as the Void. Once a single, probably man-sized individual that was convicted of some terrible crime, the being that would become Plex was as punishment cast into the Void and transformed into an enormous monstrosity, a fleshy planetoid thousands of kilometers across. The ancient criminal is not alone in its banishment, however; since its transformation, others that learned of it decided that this prisoner would make an ideal prison, and translocated other dangerous criminals there to live out the rest of their lives on (or beneath) the living world's surface. Somehow, word of Plex has spread to many worlds and even across many cosmoi, and prisoners from all manner of places have been cast there. Plex has thus become a bizarre melting pot of life forms from multiple planes and cosmoi, all of whom are exiled criminals or their descendants, and most of whom share the common goal of finding a way to leave.
History
The original nature of the criminal that became Plex remains a mystery. The fact that most of Plex's organs and features seem to be human—it does have scales and tentacles and other decidedly nonhuman features, but they are substantially less common—leads many scholars to believe that it was originally human itself, or at least very humanlike. In fact, going beyond that, the fact that most (though by no means all) of its exposed flesh is pale pink in color, and the majority of its hair dark blond, leads some to opine that these were the original colorings of the human that became the world. Some simple genetic tests have seemed to give more weight to this, as well as suggesting that whatever else it was, Plex was originally male. Still, all of this is somewhat speculative, and it's hard to know just how much weight to attach to these evidences. Perhaps it's all true, or perhaps Plex's original form was female, or genderless, and perhaps it didn't resemble humanity at all.
Where the criminal lived is likewise unknown. It may be on some obscure plane well away from the usual planar byways, perhaps one now completely depopulated, its very existence having passed into oblivion. Or it may just as well have been on some well known world, but in a long ago time that no one now remembers. Even the crime that led to such a punishment has been forgotten. Again, there is no end of speculation, but speculation is all that it is; no one really knows what it was that led to the criminal's exile. Whatever it was, it was apparently so heinous, or other circumstances surrounding the criminal's trial were so extraordinary, that a completely novel and very unusual punishment was devised. The criminal, whatever and whoever it was, was not only cast into the Void, with wards set to prevent its ever leaving that plane, but was also enlarged and transformed, twisted into a grotesque assortment of limbs and organs combined and composited in bizarre ways, a misbegotten planet-sized teratoma apparently assembled from bits and pieces from multitudinous monsters.
This all happened tens of thousands of years ago, at least... and possibly an order of magnitude or two more even than that. Whether the original civilization that condemned Plex to its fate later hit upon another use for their transformed exile, or some other people somehow discovered it and saw potential there, at some point someone decided that Plex would be an ideal place to banish other criminals to. After all, the same wards that kept Plex itself from escaping would work just as well to contain those on its surface, or within it. Word of this practice later spread, and now on many worlds Plex has been used as a place of banishment for the worst of criminals that, for some reason, those in power are reluctant (or unable) to kill outright.
Geography
On both the large and the small scale, Plex is a jumble of mismatched parts, an anatomical monstrosity. Many of the parts are human, if not human sized, but by no means all are; among the arms and hairs and tongues is the occasional beak or tentacle or other more exotic part. Plex's surface is bare pink flesh here, thick grey hide there, and in another place might have the texture and color-changing properties of octopus skin. Sometimes similar features clump together, but there are also isolated limbs and organs sprouting from random places in the ground.
Plex's strange, organic terrains leave its inhabitants no chance to forget that they are dwelling on a living being, albeit a vast and twisted one. Most of the ground is flesh; where it isn't, it's usually because it's covered in scales or in chitinous or calcareous shells, or because it consists instead of exposed bone or muscle or other normally internal tissue. Hairs are commonly found sprouting from the surface—or cilia or feathers, somewhat less commonly", sometimes small and covering the ground like grass, and sometimes growing to many times the height of a man, either standing singly or growing together into groves or great forests. Seas and lakes of blood, sweat, or other fluids occur all over its surface (though there are also some large bodies of more or less pure—or at least potable—water), and in many places recognizable—or unrecognizable—organs and limbs protrude from the surface, either sized similarly to those of a human (or even smaller) or vastly overproportioned. Eyes, tongues, claws, and even exposed lungs and other parts grow throughout the world's surface.
Not all the terrain, however, is made up of Plex's own substance; some other organisms have found a foothold there, taking their nourishment from Plex's blood and tissues. Moss and fungi grow over some areas of Plex's flesh, in places forming large forests (though in other places "forests" are created by Plex's own tissues—hairs, feathers, or branching appendages).
Etymology
The origin of Plex's name is a matter of some conjecture. A few people assume it is derived from the name of the ancient criminal who became the world, but there is no evidence for this. One common derivation is that it is a bicronym for either place of exile or plane of existence (the reasoning being that Plex, while not technically a plane, it is isolated enough from anything else that it might as well be), or possibly of both in a clever polysemy. This, however, is perhaps a bit unlikely; it's often easy to find an acronymic explanation after the fact (witness the fact that two such explanations have been found for Plex's name), but more often than not such a derivation turns out to be simply a folk etymology. Furthermore, while it's not altogether impossible, there is no reason to believe that Plex's proscriptors—or whoever gave it the name—spoke English. The derivation from the Latin plexus, meaning intricate, tangled, or (possibly) twisted, is perhaps slightly less fanciful, but still dubious at best. There's no more evidence that anyone involved with Plex's exile spoke or understood Latin than English, and in any case there's no obvious reason why it would be given a name with that particular meaning—certainly, with its immense size, Plex is intricate, and Plex is certainly twisted from its original form, but there are many qualities that would seem to be more representative. In the end, the reason for Plex's name seems destined to remain as dark as its crime and original nature.
Inhabitants
The residents of Plex come from all over many cosmoi. Virtually any world might have cast an exile to Plex; even on those worlds where Plex is generally unknown, one or two mages or other powerful persons may have learned about it and inflicted such punishment on a small handful of their enemies. Really, in fact, there are few if any worlds where exiling criminals to Plex really has been at all common, but a great many worlds where it has happened only once or twice. The result is that the exiles of Plex form a very motley company, not only in their species and cultures but even more so in their places of origin; almost never do they happen across any others who share their homeworld (unless they happen to have been exiled together). The only thing they have in common is that they were all separately banished to the same place.
The fact that most of the exiles were criminals, however, does not necessarily make them evil or antisocial. Some of them may have been resistance fighters against tyrannical regimes, personal enemies of those in power, or innocents who fell afoul of nefarious political maneuvering. Many of those exiled to Plex were decent people, who are willing to work together to make the best of it in their unwanted new home. Still, on the other hand, many of the exiles were hardened criminals who have not lost their vicious tendencies to prey on their fellows, and who make Plex a more difficult place to live. Of course, this isn't a black-and-white affair, and many of Plex's residents fall in between these two extremes... reformed criminals, mischievous but basically good-hearted scoundrels, and treacherous but law-abiding misanthropes can all be found among the exile population.
Not all of Plex's inhabitants are exiles at all, however... many of those banished to Plex have started families (or, at the very least, have procreated), and probably about seventy percent of its current residents are the descendents of past residents, and not new exiles. That the proportion is not much higher is in part because Plex is not a congenial place to raise children, and few of its residents have much interest in a family life, but perhaps more important is the fact that due to the diversity of species represented the chances are fairly low that any two random residents of Plex will be interfertile. Despite these obstacles, however, some have managed, and a few longstanding dynasties exist on the exile world, of which the Cacsis family is perhaps the best known.
Plex has its own unintelligent "wildlife", as well, though its origin isn't entirely clear. It's possible it was created when Plex was transformed, as part of its curse; it's also possible that they are descended from life forms somehow brought along when a criminal was banished there, or created by some of the exiles living on Plex's surface, as a source of food or perhaps just a side effect of some magical mistake. Some scholars have even suggested that Plex's native life has been spontaneously generated due to some of the ambient magic residual from Plex's enchantment, or that this same ambient magic has transformed the mites, fungi, and microbes that presumably lived on Plex when it was still human (or whatever it was). It has even been proposed that Plex's alogous life actually devolved from ellogous exiles. It could be, of course, that different species got there in different ways, and so more than one—and perhaps even all—of these these theories may be true.
Regardless of its origin, however, apparently native alogous life, both flora and fauna, does exist on Plex. The former includes mostly fungi, though a few strange types of tree grow on certain parts of Plex's surface. Given the profusion of projections of diverse forms that sprout from Plex's flesh, though, it's often hard to tell whether a particular efflorescence is a separate organism or just a part of Plex itself. Plex's fauna are more easily recognizable, and include, among many others, the large red ollies that suck blood from Plex through their pointed probosces, the meatworms that burrow through Plex's flesh, and a diverse group of large arthropoid called baams.
Society
Unsurprisingly for a place populated by exiles and their descendants, Plex has no overall government. Some parts of Plex have organized into small nations of sorts and city-states, but large areas of Plex are still more or less in anarchy, though even there some powerful individuals have risen to positions of unofficial leadership. In fact, some of them have developed baroque hierarchies and organizations to control the people under them. To confuse matters further, however, there is a fairly high rate of turnover among these local leaders, as they are supplanted by underlings or eliminated by rivals; long-term alliances with the people in power can be dangerous on Plex, since one never knows how long they're going to stay in power, and how their successors are going to feel about their friends.
Resources are not in short supply in Plex; its inhabitants live on their world in multiple senses. The flesh of the world itself is edible, and plentiful, slowly regrowing if damaged; and some of the organs that grow from it are likewise edible, and give some variation to the inhabitants' diet. (Whether this harvesting of the substance of the world causes it physical pain—as it quite likely does—is a matter that few bother to worry much about.) Furthermore, many of the fungi and (rare) plants that grow on Plex's surface are themselves edible, as are many of the weird animals that feed in turn on the fungi or on Plex itself.
Perhaps in part because of this abundance of resources, and in part because of the chaotic government (or lack thereof), trade has never developed much on Plex. Certainly some local barter goes on, and some few people have even managed to make a living as itinerant salesmen, but there are no widely used trade routes or major industries. For that matter, food and drink being so plentiful, many of Plex's inhabitants don't find it necessary to work for a living at all... which frees up more time for them to jockey for position in the society's byzantine politics.
Escape
Leaving Plex is, naturally, a very difficult process—otherwise, it wouldn't be much use as a prison world. Plex features a sort of integrative magic similar to that active in many compits; its inhabitants tend to find that whatever magic system they are used to seems to work on Plex, with minor modifications. However, magics that would allow egress are an exception; all known means of interplanar or intercosmic travel fail completely, sometimes without any result at all and sometimes with a result very different from what the would-be traveler intended... such as translocation to some other spot on or within Plex, or, in more unpleasant cases, such as the unfortunate subject's matter being scrambled into a gloppy mess. Repeated attempts have been made to create a special spell specifically for the express purpose of leaving Plex, but thus far no such attempt has met with success (or at least, if it has, the inventor has used the spell immediately without spreading word to others).
Persistent rumors exist of hidden portals off of Plex, or special paracarminical magics that can do what known spells and talismans apparently cannot. Again, however, if such portals or magics do exist, their discoverers have kept them a secret, and while many people have gone missing on Plex, there are plenty of other possible explanations for that. Still, the search for some of these hidden exits goes on, fruitless though it may be, and rumors continue to spread. One of the most popular accounts is that somewhere a sort of tether still ties Plex to his former plane, and that if it could be reached—by traveling somewhere deep into Plex's interior, perhaps somewhere in the vicinity of Plex's umbilicus—an exile could follow it back and make his way out of the Void. Naturally, there's no more evidence of this tether really existing than there is for any of the other myriad rumors about similar hidden escape routes.
There is one way that it is possible to leave Plex, but only briefly and without going far. Nothing prevents those with the ability of flight from just going straight up from Plex's surface into the emptiness of the Void. There are several reasons, however, why this is not generally considered a good means of escape. First of all, the integrative magic system active on Plex stops a few kilometers from its surface, so past that point magical flight is useless, and anyone without some non-magical means of propulsion (or enough velocity built up to surpass Plex's escape velocity) isn't going to get very far. Even if one does manage to get away from Plex's surface, the curse of exile remains in effect, and it's still not possible to leave the Void—even through portals or other means that work perfectly well for anyone who hasn't been to Plex. And even the fugitive's sojourn in other parts of the Void will be short-lived, because the curse also calls him back to Plex; anyone who has managed to leave Plex by this means finds himself translocated back to a random spot within Plex within a matter of a few days.
Worse still, though, are the strange transformations that sometimes occur to those who stray too far from Plex's surface. What exactly triggers these effects is unknown, and it may be entirely random, though there certainly has been no concerted effort to study the matter. In any case, those who venture up too far are sometimes subject to agalmatation, sometimes accompanied by size change or other transformations. These enchantments prove very difficult to break, furnishing all the more reason not to try to escape Plex by this method.