"Vimes took the view that life was so full of things happening erratically in all directions that the chances of any of them making some kind of relevant sense were remote in the extreme."
- – Terry Pratchett, Feet of Clay
It is the plane of ultimate Chaos and randomness, where matter and thought collide, allowing for infinite possibility.
Though unlike THAT other realm of Chaos that we all know about, Limbo is FAR less hazardous to your health; your physical body will not dissolve on entry and neither will your soul immediately be twisted into something to serve the whimsy of Dark Masters.
Geography and Traits
As a rule, Limbo does not have a discernable geography. The plane is primarily made up of "Chaos Matter" which is an endlessly roiling and toiling stuff of no constant physical substance or appearance.
A pocket of air might suddenly congeal into water after a round, harden into stone after three more rounds, then spontaneously ignite into flame after two rounds, before bubbling into water again in less than one round... There's no particular pattern to what might happen next, and it might give you the impression that Limbo is inhospitable.
Thankfully, Chaos matter can be shaped into something else merely by willing it into something else and there exists guilds of "Anarchs" who spend their days doing just that. At the most basic level, any person can create a pocket of stabilized air for them to breathe without any fuss. More complex combinations of elements can be created with stronger minds to hold it together, allowing individuals or groups to create virtually anything imaginable and imbue their creations with any trait they can conceive of... From a field of rainbow unicorns to a river of boiling piss.
This makes travel fun, as areas dominated by "Air" allow users to fly (or fall) in a chosen direction, where swimmers could create "Water" for themselves and move that way. You could also go more complicated and build a road for yourself if you simply wanted to walk somewhere.
Those areas of controlled Limbo last as long as the creator maintains his interest, or unless someone else is around to maintain control for him. Otherwise it all just melts back into chaos matter, so woe betide anyone who falls asleep in his stabilised area without someone to keep watch.
The most powerful minds can imbue a level of permanency on the chaos-stuff, so they don't have to worry about it dissolving when they stop paying attention. Nor do they need to be concerned about someone else's idle thoughts punching holes in it accidentally, so it is those areas of stable matter that population centers can most often be found.
However, it should not be understated that the plane is Strongly aligned to Chaos, so if you're a structured, well ordered individual (i.e "Lawful") then your psyche might crack under the pressure of all the nonsense that the plane puts out, coming with a corresponding reduction in your ability to control the environment.
While the laws of physics are always in flux, the laws of magic are equally affected: Spellcasting becomes hazardous in all areas other than those permanently stabilised, creating a chance that any time you cast a spell it might fly off in a random direction, do nothing at all, do something entirely different, or go Nuclear. That's just the nature of Chaos, so don't say you haven't been warned.
Limbo Petitioners, like all petitioners, are those who came to the plane as their reward in the afterlife, their souls take on the form of self-controlled chaos matter in death and typically have no sensible shape or appearance, they are as equally likely to exist as a talking cloud one minute or a twisted knot of rope the next, then set you on fire for the privilege of eating with their spoons (?), and then they sta--... [looks into distance] huh? [runs off] Pancake! PANCAKE! PANCAKE! Two can be observed in their natural state here. The director of the movie "Rubber" was most likely one of these. DMs who need help roleplaying one are advised to google "Florida Man."
...thankfully they are quite rare, for a few reasons. Firstly, because it's a bizarre individual who solely embodies the alignment to THAT degree of Chaotic Stupid to end up in Limbo. Most Chaotic Neutral individuals end up in Ysgard (If they were hedonists or thrill seekers) or Pandemonium (if they were nihilists or "actually" insane). The second reason is that in a realm of infinite space, a creature of pure chaos, uninterested in the affairs of others, is likely going to strike off on its own and never be seen again.
The Slaadi are the "Natives" and embody Chaotic Neutral as much as Archons and Demons embody their own alignments.
Githzerai are cosmic immigrants from a long time ago, they could've picked anywhere in the multiverse to settle down but they chose here for the sake of a good challenge and started stabilising the environment to suit their needs.
Limbo is synonymous with the Maelstrom in the Pathfinder Great Beyond cosmology. The two planes are thematically identical in that they are both Chaotic Neutral rolling expanses of raw potentiality that can be shaped into anything conceivable, though because the Pathfinder cosmology is structured differently, the location is different, as is the plane's history.
The Maelstrom is typically considered to be the oldest plane in existence; surrounding and encompassing everything else, all other planes being classic instances of order arising from chaos; when mortals arrived in the Prime Material their thoughts impacted the raw chaos matter and spawned the other planes. Instead of being positioned directly opposite to the plane of Axis, the Maelstrom has its borders right up to the walls of the city of pure law (as well as all the other planes, as stated previously), and assails them relentlessly, although this "war" against law is not going well for the forces of chaos, as Axis continues to grow outwards; annexing more territory from the Maelstrom all the time. Though law may grow infinitely, the chaos is infinite by itself, meaning the conflict may continue up until the end of time.
Instead of the non-OGL Slaad, Paizo have gone with the serpentine Proteans, who fulfil the same role (though they manage to one-up the Slaad in that even their own bodies are mutable, to the point that they change genders on a whim like a tumblrina), being mostly hostile to all other forms of intelligence, and generally being belligerent bastards. Also because there is no Ysgard or Acheron in the Pathfinder variant, the Maelstrom is where the Valkyries, Einheriar and Orcs have settled, making the plane the realm of endless battles.
Though if there was an opposite end of the Outer Planes, then the ever growing Spire of quintessence from the Boneyard probably exists at the farthest point away from the Antipode of the Maelstrom; essentially the sinkhole of the multiverse where all quintessence gets sucked up and redeposited in the Positive Energy plane in the Inner Planes, essentially keeping the cycle of souls running. The Antipode is also one of the few places where the Proteans do not gather, as the area is protected by the Aeons; the race of Neutral Outsiders who protect the balance, essentially stopping the Proteans from doing something unbelievably stupid like corking the sinkhole and stopping life from being recycled, or stretching it wider and causing the outer planes to turn themselves inside out.
Unusually, the Maelstrom is not exclusively home to chaotic individuals. Many true neutral deities not concerned with life and death, including Nethys, make their divine realm here. The city of Basrakal is populated almost entirely by outsiders of the "wrong" alignment for their type.
|Great Beyond of Pathfinder|
|Astral Plane-Ethereal Plane|
|First World-Inner Planes-Shadow Plane|