To see how the Planes of Chaos fit into the rest of the Multiverse, see the entry on the Planes of Existence. The Planes of Chaos, sometimes known as the Cosmic Abyss, are a number of planes of existence roughly arranged into a stack. The further away from the Material Plane, the more alien, bizarre, dangerous and horrific the planes become. Demons, the manifestations of Chaos, are found on the Planes of Chaos, but are by no means the only inhabitants - indeed, on the closer planes such as Hestoris, Carceros and Aegothis demons are quite rare, outnumbered by more conventional chaotic creatures found on the Material plane.
Hestoris, the First Plane of Chaos
Hestoris is one of the most easily accessed planes of Chaos. Compared to some of the horrors found on the deeper layers of Chaos, Hestoris is almost mild. It is basically earth-like, but always with a cloudy overcast sky. There is daylight but no direct sunlight.
The terrain ranges from subarctic to temperate, with forests and swamps dominating, with a few ranges of hills and some open plains. There are some large lakes and rivers, but no known seas or oceans.
The flora and fauna are like Mystara but with more monsters and predators than the herbivores should support. Unnatural monsters that are neutral or unaligned on Mystara and Kaelaross, such as griffons, hydrae and carrion crawlers, have a chaotic slant to their behaviour here on Hestoris.
The intelligent inhabitants are mostly chaotic humanoids that have formed large tribes. Goblins, orcs, gnolls and ogres dominate but lots of hobgoblins, kobolds, bugbears and trolls are also found. There are some centaurs, lizardfolk and rakastas but these normally-neutral folk have turned to chaos since dwelling on this plane.
The land is dotted with ruined ancient cities that have been taken over by the chaotic humanoids. Although the humanoids dwell there and occasionally repair or modify the cities, it is clear that what or whoever built the cities were considerably more sophisticated and organised than goblins and orcs.
There are some humans, halflings and elves who have formed refuges safely distant from the cities, forming makeshift villages and camps up to 200 strong. Due to wandering orc and gnoll armies, the humans and demihumans have to be able to move home within a day or less. Most of these folk are neutral rather than lawful. Hunting, gathering and fishing is their way of life.
There are some demons on Hestoris, but they are uncommon and held in reverence by the chaotic humanoids. Although capable of wanton mayhem, these demons are often on a mission to hunt down and kill lawful creatures (including PCs and NPCs).
The elves of Hestoris have an oral tradition that says that over a thousand years ago Hestoris was a world on the Material Plane. But when a terrible plague struck, many, but not all, of the inhabitants turned to the Chaos Gods to save them. Hestoris was torn from the Material Plane and became a Plane of Chaos. Then the Chaotic races began to wage war, the sunshine stopped and the demons began to appear. The Chaos Cultists of Kaelaross have heard this and are forming ambitious and terrible plans.
Carceros the Eternal Dungeon
Carceros is a plane of Chaos that is a huge dungeon. Carceros has a number of links to the Elemental Plane of Earth, and some creatures from the Plane of Earth are also found in Carceros.
Carceros has numerous levels, areas and regions, nearly all of which are interconnected by stairs, long passages or portals. There is considerable variation between areas - one area might be natural limestone caverns, another the well-furnished lair of decadent races. Another might be a bone-filled catacomb or else a prison with guards and inmates. Each area may be of any level the DM considers suitable, and the inhabitants tailored accordingly.
The inhabitants are generally the same as dungeon dwellers on the material plane, but with more demons and earth elemental creatures. As is true for Hestoris and other planes of Chaos, creatures that are neutral or unaligned on the Material Plane are often chaotic here. Although the DM should decide, it is suggested that treasure on Carceros is reduced from the amounts in "normal" dungeons - this is a Plane of Chaos, not a bank deposit.
Conditions on Carceros are similar to most material plane dungeons - very dark, damp, some pools of water (the larger ones inhabited by creatures), stale but breathable air and normal gravity. However, there are 2 differences:
1) Food is less important - only a third of the normal amounts of food are needed, and starvation takes three times as long to take its effects. This allows a larger number of predators given the limited fungus-based ecosystem.
2) Movement Spells including Passwall, Teleport, Dimension Door and Find the Path all have a 50% chance of failing. The Plane does not easily allow shortcuts to bypass its perils.
Some areas of Carceros exhibit bizarre and impossible geometries, like Tesseracts, Mobius Loops and Escher-style ever-ascending stair cases.
Urdunor: This plane of Chaos is volcanic and fiery, with great black basalt and granite plains and mountains pock-marked with deep vents and split by chasms from which searing lava spews. Red dragons, fire giants, hellhounds and similar monsters roam. In particular, the Efreet have a mighty empire here centred on the legendary City of Brass. There are thought to be many portals to the Elemental Plane of Fire.
Aegothis: Also known as the Eternal City, this plane is a massive (infinite?) metropolis where all the architecture and engineering is decaying and crumbling, while all the inhabitants are evil and cynical humans or worse. Werewolves, wererats, vampires, dopplegangers and devil swine are all found hidden among the population. Deception, greed, treachery and murder are the norm here.
Toldaar: This plane is a massive ocean dotted with many islands. In the ocean there are many types of carnivorous monsters, including sharks, dragon turtles, sea serpents and octopi. The islands are not much better. Most are covered in dense jungle and inhabited by all sorts of reptiles, including dinosaurs, snakes, lizards, hydrae, lizardmen, crocodiles and dragons (especially black and green ones). Similar to Urdunor, deep in the sea are portals to the Elemental Plane of Water.
Noreesis Vale: This plane is icy cold and shrouded in the darkness of midnight. In the bottom of the valley there are clumps of hardy vegetation which woolly rhinos and mastodons feed on, but beyond that, especially on the slopes and peaks, it is an icy, desolate place. Numerous ice-dwelling creatures are found here such as frost salamanders, frost giants and chaotic white dragons, while the environment is just as deadly as the denizens. Avalanches, ice chasms and altitude sickness can all catch the unwary traveller, assuming the frostbite and hypothermia don't kill him within the first half hour.