As often happens with a plane tending towards chaos, giving concrete numbers on the Burning Hells is akin to nailing gelatin to a wall. The size of this place is indeterminate, though it's a safe bet to say that it's within 10% of that of the High Heavens (I say this because my measurements always came up with something in that range, though it was never the same number twice).
The hallways of the Hells are made of fibrous red material streaked with black that bears a strong resemblance to diseased muscle tissue. Bone and ebony support struts, spikes, and random protrusions are mixed in all over the place, generally giving it a foreboding atmosphere. Dim red lighting comes from nowhere in particular, and deep shadows are quite common.
The layout of the Hells is best described by an analogy. Imagine someone on hallucinogenic drugs taking a plate of spaghetti and twisting it into a Klein bottle. That's the Burning Hells. The largest open space in the entire plane is the Well of Souls, and the next largest are the throne rooms of the various Evils, each no more than a hundred meters across. The entire place is honeycombed by tunnels and corridors going off in every direction. "Down" is relative, and changes from tunnel to tunnel and sometimes within the same tunnel. At one point my ultra-shielded backup-backup sensor (I went to the Hells after the Heavens) registered that there were seven and a half cardinal directions. Then it dropped to the normal six, then spiked up to nine, then to four, then back to six, all within the space of ten seconds. As if that were not enough to get you thoroughly lost, the Hells wrap back on themselves just like the Heavens, except there's no nice bedrock to tell you when that happens. Walking through the tunnels, they seem continuous, though its enough to drive a locator mad since the distance from point A to point B changes every minute as the tunnels rearrange themselves and warp space-time to join up elsewhere. An apparent shift from within the tunnel of two meters can end up connecting the exit to a place halfway across the Hells. Basically, if you plan on vacationing somewhere, this ain't it. Though if you have a sufficiently strong will, you can force the Hells to create tunnels to where you want to go. They are rarely quick and if you let your concentration lapse they can close up on you, but it's one way to get around.
The closest thing to a center the Hells have is the Well of Souls. The name refers both to the 16-kilometer-wide cavern and to the Well itself, which is a slightly smaller ball of roiling gray soulstuff. Unlike the Firestar, I could actually get a reading on this and found that it is the repository for the souls of the anchor world that fit into the "evil" category (like the High Heavens, these souls are not reborn as fiends, except in the case of undead). And from my readings, it's definitely one of the worse ways to spend eternity. What it amounts to for the soul trapped there is sensory deprivation for all time, barring being yanked out by a summoning, resurrection, or reanimation spell. The local humans who think Hell is fire and brimstone don't know the half of it, since being totally and completely alone with yourself for eternity is much, much worse than any physical torture (if you don't believe me, jump in a sensory deprivation tank for a bit. Your average human can last about 24 hours before he/she begins to crack).
The beings native to the Burning Hells span just about anything you can imagine and several that you can't. Things of all shapes and sizes lumber, scamper, slither, and chitter down its hallways, usually armed to the teeth with, well, teeth. And claws, spikes, horns, and other things designed to tear things like angels apart. Few of the fiends have offensive magical abilities, probably due to the high magical resistance of the angels. Most fiendish spells deal with fire or outright destruction, and are red or black in color when cast.
Only one trait can be said to span the entire Hells, and that is mutability. A sufficiently disciplined mind can take a fiend and impost his own will on it, forcing it to change shape, abilities, and pretty much anything else to whatever you want. The larger the change, the harder it is to morph a fiend into it. Intelligence, for example, is an extremely rare natural trait for a fiend, and the effort required to boost that of one is such that the Hells rely much more on brute strength and numbers than tactics and planning. Often the higher-ups in the Hells won't bother to change a fiend at all, but simply wait for something they like to appear and then use it as a template to spawn more of them (it is much easier to force the Burning Hells to create something that already exists than to create the new fiend yourself).
Fiends basically appear more or less at random times and places throughout the Hells. Their types are often completely random assemblages of nightmarish shapes, and occasionally totally new to the Hells(see above paragraph). This probably comes from their origin in the nightmares of humanity, where that which is not seen is more frightening that that which is, so human imagination gives shape to things worse than any fiend could think up itself. On the relatively rare occasions the Hells interact directly with humans, this lets them tailor their fiends to produce maximum fear. I actually caught a scan of a few floating areas of Potential, things that are not even fiends until a mind looks at them. They don't last long before turning into normal hellspawn, but it's a very useful thing to have available.
The population of the Hells is somewhere between 300 million and one billion hellspawn; I could not get more accurate than that, what with all the shifting of the corridors and deaths and spawing of fiends. I would not be surprised to learn that it fluctuates between the two in as little as a day's time.
When a fiend dies, its soul-analogue is absorbed back into the Burning Hells itself, which can be considered a gigantic, living entity (though not sentient by any means). The fiendish soulstuff is then recycled and spat out in the form of another fiend, possibly bearing a resemblance to its last incarnation but probably not. The hellspawn thus spat out can be anything from an evil flatworm an inch long and with all the dangerousness of a drugged sloth, to a mutli-ton behemoth with more jaws and claws than you can count and armor a meter thick.
The Hells itself can turn against an intruder. I took out a sample leaf from the High Heavens and tossed it to the ground at one point. Instantly, several new openings appeared in the hallway's sides, and within seconds dozens of fiends were tearing at it and at each other in order to destroy it. This is probably one of the reasons why the angels never make forays into the Hells, while the fiends try to attack the High Heavens directly (and fail miserably, of course. They just don't seem to realize that attacking an enemy's stronghold amounts to suicide).
There is a spectrum of intelligence and cunning as you go up the ranks of the Burning Hells. The lower you get, the more chaotic the fiends are, while the higher you go the more they are able to plan and prepare. Even at the level of a Prime Evil, though, they often succumb to their baser natures and run full-tilt into something without considering its consequences. Sheer numbers and determination manage to keep them going in most cases.
Any attempt to classify the fiends of the Hells would send a taxonomist into conniptions. But there are a few broad categories that might be worth knowing:
Though technically not products of the Burning Hells, only this side of the Sin War actually uses them. They are formed by taking a corpse and yanking the body's soul out of wherever it is now (be it the Well of Souls or elsewhere) and sticking it back it, with a heavy magical restraint making the undead a servant of the raiser's will. If the soul cannot be retrieved for some reason, a random one from the Well of Souls is taken instead. Conscious memories of the Well are lost in this case, but enough remains to make an undead fight like crazy to keep from going back.
Within the undead there are several levels, ranging from brain-dead zombies to vampires and beyond, spanning the whole range of intellect and strength. On average, though, a reanimated undead is stronger than the body was in life, though not necessarily quicker. A few versions of undead (like vampires) get special powers, which vary from type to type and sometimes even being to being.
These are pretty scarce, since few angels captured by the Hells manage to stay in enough pieces to create a fallen angel. The change is so drastic that they cannot even be qualified as angels, but rather as a demon that has taken over an angel's body. It takes a very powerful fiend to effect the change, and it can only happen to a cherub. Anything higher than that and the attempt of change destroys the angel.
Any abilities can be lost or gained during this process, including flight, magical resistance, magical powers, fighting prowess, etc. Fallen angels gain the mutability of the Burning Hells, and their appearance always shifts to something distinctly non-angelic. Often the wings turn a black flecked with green and writhe almost like snakes, while the armor tarnishes, grows spikes, or changes material entirely. A fallen angel's sword always has black flames, and it is one of the few fiends that can harm an angel through physical contact (with a seraph or cherub it feels no pain, an angel and it experience equal pain, and it would come out far the worse with an archangel).
There are seven Evils, four Lesser and Three Prime, which are anthropomorphic representations of the local human's ideas about evil. They are avatars of Evil in a similar way that a son of the morning is an avatar of Law, but to a slightly lesser extent. They are still plane-bound, though in all probability none of them know this since they aren't the type to explore extra-planar options with the angels right next-door.
The four Lesser Evils are named Andariel, Azmodan, Belial, and Durial, and they are the personifications of anguish, sin, lies, and pain, respectively. The three Prime Evils are Baal, Diablo, and Mephisto, constructs of destruction, terror, and hatred. The Primes seem the more ambitious of the lot, or at least those more likely to tick off the others enough to get banished to the anchor world for a while. They each have some sort of attachment to magical artifacts called "soulstones," but I was unable to get more than the name regarding them.
Each of the Evils has personal dominion over a portion of the Hells, as defined by how far they can stretch their minds and force the Hells ro obey them. Powerful as they are, I do not think they control more than 10% of the entire Hells together, the rest of it existing in a kind of feral "kill and be killed" state. They often send scouting parties into these areas to look for new breeds of fiends, as they are far more likely to appear there than within their own dominions.
Sons of Perdition
Having the Evils at the top of the Hells' pyramid did not seem right to me, when there are around 500 sons of the morning up in the Heavens, and each one of them is more than a match for any one of the Evils (or any two or three of them. Four's getting into iffy territory). These types of subplanes always have a balance of sorts. It's like an unwritten rule (or perhaps written, but if so I've never seen it). So I set my backup-backup sensor to maximum gain and started looking.
After a few minutes, I found one. These beings have no names, but I call them "sons of perdition" as a counterpart to the sons of the morning. They are avatars of chaos and destruction, just as much as a son of the morning embodies law and order. A bit of reflection on this reveals why they were so hard to find.
As embodiments of pure chaos, they cannot have any sort of form whatsoever and, upon their spawning, immediately tear themselves apart. Your average son of perdition has a lifespan of four picoseconds, such a short time they don't even have a chance to do anything. A being standing right where one appeared would probably experience disorientation, but little more. For being the most powerful entities in the Hells, they don't pose much of a threat. Still, I would not recommend trying to get one's attention via magic or beacon or what have you. I got a reading on one that managed to last an entire millisecond before destroying itself. Every bit of the Hells within ten meters was torn to existential tatters, and it took over half an hour for it to pull itself back together.
The Hells don't pay much attention to the anchor world, mostly because it takes so much concentration and intelligence to do so, and most of the fiends are severely lacking in that department. The higher-ups, most notably the seven Evils, have their eyes set on the world, though, and they are the ones waging the Sin War. For the most part they use subversion like the angels, though "coercion" is probably a better term. Out-and-out possession is not uncommon, either.
The other subplanes have the Hells basically marked as a "Here there be dragons" realm, a place where it's safest not to go if you want to remain in one piece. I don't blame them and wish I could do the same, but I'm paid to stick my neck out for these things, so I've kind of got an obligation to it.
In a strange counterpoint, several major religions have a sort of anti-god figure responsible for evil, the most common name being "Satan." The Hells think contemptuously of anyone who tries to assign him to their realm, and will probably kill you for your insult (if they were bright enough to understand it. Then again, if they weren't, they're going to kill you anyway). The being himself was unavailable for comment.