Baator
Baator, also known as The Nine Hells, is one of the Lower Planes in the Great Wheel cosmology (and resides in the Astral Sea in the 4e cosmology). Few names evoke as many dramatic and terrifying images in the minds of mortals as The Nine Hells, and with good reason. Baator personifies the epitome of the Lawful Evil ethos as a place of punishment, oppression, and endless, soul-crushing conformity. It is the eternal destination for those who force their will through unjust violence, tyrants, cruel overseers, malicious judges, unscrupulous barristers, and (most) politicians. Basically, anyone who dominates others through authority or manipulates the law at the expense of others will probably find themselves on a one way trip to the big H-E-Double Hockey Stick when they get their ticket punched.
Traits
The plane has normal gravity and time characteristics. The layers get smaller the lower you go, and attempting to fall or fly off into the space outside them gets you destroyed by some kind of force (described as a "gnawing vacuum"). There are, however, several paths allowing walking to the lower layers, and there are places where you can fly through layers (notwithstanding flying patrols fucking you for trespassing). Deities can warp the plane in their vicinity as needed, but few do much more than alter their personal areas (since interfering with Asmodeus and his works is something even gods are sketchy about). Other than some penalties to Cha checks for Chaotic and/or Good beings, nothing really inhibits magic outright, and only a couple of the layers have any elemental traits that make them outright hostile to standing there.
That's the overall traits. Each layer, however, is very unique and has a myriad of dangers. For example, Avernus has the above traits, and you may think it would make for a pleasant little romp through the planes. That's how you end up in the dead-book, though: there's the fucking Blood War to deal with (either staging for an attack into the Abyss, or repelling one from there), the River Styx (which has its own unique dangers), and the fact that medium-strength fireballs fall randomly on the layer (though there are allusions that it's not entirely random). Just dealing with the press-gangs, patrols, and other bullshit on the first layer is an adventure unto itself; things only get more challenging as you get to lower levels where more powerful devils will challenge pretty much everything you do or say.
Note that each layer gets progressively more dangerous for various reasons. By the time you get to Nessus, you're dealing with a being that knows your every move on his layer, and armies of pit fiends (as in, pit fiends are the regular fucking soldiers down here), as well as legions of unique devils seeking favor, cultists who serve the King of Hell directly, and pretty much anything the DM feels like flinging at you, like a half-fiend great wyrm red dragon cleric of Tiamat (who's just down to deliver a gift or message for Tiamat's gracious landlord).
Geography
Just as in "The Divine Comedy", Baator is arranged in nine layers. Each one is ruled by an Archduke, a unique devil that is granted their status by Asmodeus. While the Archdukes aren't deities by any means, they manage to use various methods to alter the layers to their tastes and necessities. There are pacts with Lawful Evil gods, the use of slaves taken from across the entire cosmos, and powerful magic that is the envy of every wizard in creation.
Getting from one layer to the next isn't physically hard. There are paths that let you walk down to the next layers, and flight can really help with all of this. The real trick is avoiding the guards, patrols, etc. that are meant to keep devils and petitioners in their proper place... and to keep outsiders, well, out of the layers. Trespassing a lich's necromantic sanctum may get you killed, but doing the same thing in Hell gets you enslaved or tortured or worse.
Aside from the cities and strongholds of the devils, there are a few other unique landmarks. The deities Tiamat, Kurtulmak, Hecate, Set, Sekolah, and possibly others have their own personal domains among the various layers. The City of Dis actually ranks as one of the great planar markets, since devils have great need of armaments and war material for the Blood War. If you are brave (or foolish), you can even try to find the fortress guarding the Pact Primeval (and possibly tap some of that vast power of Law to use to fight creatures of Chaos).
4th Edition D&D changed the layout of the Nine Hells, so that instead of resembling a stack of floating pancakes, the Nine Hells can be seen from the Astral Sea like a single planet, with the visible "surface" of the "planet" being the first layer, Avernus, and the other layers now arranged as smaller spheres nested inside each other, with the final layer Nessus situated on the underside of the Eighth layer Cania, facing the "planet's" fiery core.
Avernus
The first layer of Baator is a wasteland covered with the remains of millions of creatures who died in battle there, while fireballs literally rain down on the area. Fresh blood and gore help you to slip and cut yourself on jagged, sharp stones, and the dried bones, horns, and other remains of defeated creatures crunch underfoot. The River Styx passes through, with little side creeks and streams running off down into the lower levels, while marraenoloths (the "boatmen" of the river) ferry collections of fiends in various numbers to some business, either battle or soul-harvesting.
It is really one of the most accessible of the Lower Planes to get to, at least in terms of not needing a lot of special magic or other stuff to get there: there are links with the Elemental Plane of Fire (the devils do brisk, good business with the efreet); there are links in Hades to a number of Outer Planes (as well as the Astral), allowing you to go "up" river to Avernus; you can always try to hop the portal in the Outlands town of Ribcage (either legally or through trespassing, whichever you feel like). This lovely first layer is where the Lawful Evil souls not pledged to a specific deity end up washing up on a half-mile part of the Styx called the Shelves of Despond, where they are herded up and either marched off to the Maggot Pit (where souls are eaten by huge maggots and then shit out as lemures; this isn't poetry, that's literally how it happens), or sent away to be transformed through some gruesome process or experiment into something even less fun (such as being stripped of all mental function and turned into some kind of useful object).
This is also a continent-sized battlefield of the Blood War, so there's a lot going on. You've got several fortresses run by this and that devil commander, occupied by a force of demons, or in ruins and being plundered by various scavengers of the planes. Groups of devils ranging in size from small patrol to a full legion of armed devils roam the area, and if you snuck your way in rather than getting good, official papers saying you have business there, it is better to stay out of their way; unlike the Abyss, a unit of devils going missing will definitely lead to some questions and investigation, and nothing pisses off devils as much as fucking mortal assholes coming in and mucking about in their great works. Never mind that you also gotta be careful about demons, who would just as soon rip your head off and defecate in the neck-hole as they would look at you, or yugoloth mercenaries who take a liking to anything you have that they want (since their contracts may specify they can "keep what they kill"). All in all, getting into Avernus is easy, but moving through it is like trying to navigate a war-front for a coffee and a pack of smokes.
Other than the Archduke, there's two groups of beings that have a lot of stroke out here. The first is the Dark Eight, a group of pit fiend generals who Get Shit Done, and in fact like the position because it's one of the highest you can get until you become a unique devil (a status that only Archdukes and/or Asmodeus himself have to give you), and the fringe benefits are pretty good: lording over other devils, even some lackeys sent by Archdukes (remember, the Blood War and keeping the demons at bay is infinitely more important than any political bullshit from the other layers); they get to exercise that Evil axis of their alignment by killing the shit out of everything that isn't a devil on this layer (and maybe a few of them, too, if it's justified); they can start building a political power-base by bringing this and that devil into their service on the front lines, or they can eliminate a political rival much the same way (by putting them into really dangerous shit to get them killed). While a lot of devils toady to the political hierarchies or engage in other intrigues, more than a few started off as lucky and successful veterans of the Blood War who clawed their way up in rank and got recruited elsewhere for efficiency.
The second group of devils to keep tabs on are the various outcast devils, also known as the "rabble of devilkin". These consist of various unique devils who are close to the power of the Archdukes (including a few former Archdukes here and there), and got fucked out of their positions and thus have nowhere else to go but Avernus. There's a huge list of these devils, and include familiar names like Moloch, former ruler of Malbolge who was sentenced to death for plotting a nearly-successful scheme to overthrow Asmodeus. There's a few other interesting outcasts as well, and it's best to walk a fine, polite line with all of them, because some of them may still be working off a few centuries of anger about their fate.
There's a path leading down through the ground to the next layer, Dis, inside of Tiamat's layer. The evil bitch was tasked with guarding that path in exchange for room and board in Hell, so abishai and actual fiendish dragons keep a watch here. Interestingly, because of Tiamat's great greed for wealth, it's not that hard to bargain your way through, just be careful how you speak to the guards there. You can also find Draukari, the realm of the kobold god Kurtulmak, and his little bastards, doing nothing but digging mines and making traps. Kurtulmak doesn't like intruders, so he just slays devils under a certain station outright, keeping most devils from even trying to bother him.
Bel has been the Archduke of Avernus for a very, very long time. This is largely due to the fact that he is happy with his position and enjoys fighting the Blood War, something the other Archdukes don't really care to deal with. As a result, his only real political schemes are to keep the Dark Eight happy, which he does by using super-intelligent planning and caution when fighting the demons; his victories have pretty much kept things happy for the devils in his command. However, as of 5th edition, the unique devil Zariel has ascended to Archduke. Zariel started as an archon sent to keep tabs on the Blood War, and saw how valiantly and skillfully the devils were at keeping the seething hordes of the Abyss at bay. She basically got the idea that the forces of the Upper Planes could sweep the whole lot of fiends if they tried, was told to STFU and do her job, got salty and ragequit to become a fallen archon and a new champion of Hell. She's apparently the exact opposite of Bel, all fired up to charge outside, kill demons and curbstomp anyone who contradicts her. For his part, Bel patiently stepped aside, and is basically allowing Zariel to inevitably fail with one of her reckless ideas, so he can take back control. Given how things go in the Lower Planes, it probably won't even be that long.
As an aside, as of 5th edition (Mordenkainen's Tome of Foes), there's been trouble on Avernus. It was overrun with demons for a while, due to some politics delyaing reinforcements, but is now again in infernal hands (claws?). It is currently even more wrecked-looking than previously, and several devil strongholds were smashed in the process. The devils there are now on higher alert than before, so trying to deceive or sneak past them is more treacherous.
Dis
Dis is both the name of the second layer of Baator as well as its principal city. The two are used interchangeably, as the "City of Pain" sprawls across a blackened iron wasteland to the point where it is difficult to determine exactly where one ends and the other begins. Black mountain peaks thrust into the ash-green skies all along the horizon, although no one can tell if they are a geographic feature or simply a mirage brought about by planar metaphysics. Within the city itself, the iron walls smoke with intense heat, causing painful burns to any exposed flesh. Once you are inside, the city is effectively infinite in size due to the way space is warped there, and short of magic you can't leave the city without devilish permission. The air is filled with the acrid stench of hot iron and the screams of the miserable souls who are condemned to spend eternity building, tearing down, and rebuilding the city with their bare hands.
A type of specter unique to Dis is created by a unique condition called "anagnorisis", where a soul has a revelation upon their death to know and understand the evil they did to others. If the soul sincerely repents soon enough, they are given a second chance as a unique race called the Hellbred, who get one last final shot at life to redeem themselves; if they hesitate for even a moment too long, they are damned as a specter of Dis. This form renders them immortal, and because they suffer emotional/mental torment forever, their suffering can be harvesting more efficiently than the usual torment of damned souls (using arcane rods on the towers and bridges of the city). This little secret is what makes the Archduke, Dispater, super fucking rich in Hell: he can sell one of these specters for several regular souls, and/or any other amount of compensation deemed "fair" (by Dispater, of course). Other devils would pay a mint for the trick of harnessing such souls and their torment, but for now it is a safely and jealously guarded secret.
The Lord of the Second rules the City of Pain from an enormous citadel of iron and lead known as the Iron Tower. From here, he oversees the ceaseless toil of petitioners while lording over his vassals with an iron fist. Mirroring Dispater's omnipresent paranoia, the Iron Tower always seems to be one block away due to spatial warping unless you have a Devil guide or magical means to truly approach the structure. A similar effect can be seen in how the City of Dis is visible on the horizon no matter where you look. The City itself also has a location named "God Street" where any Lawful Evil deities (including homebrew ones) not powerful enough to have their own godly realms can be found. There's also the Garden of Delights, a vast illusory pleasure-palace used to sucker mortals into becoming Lawful Evil (after which they suffer a prompt and entirely unforeseeable accidental death), run by a cabal of efreeti who are handsomely paid for their services. There's also Mentiri, a prison which holds two kinds of guests. The first prisoners are mortals captured in Baator, which is a mixture of do-gooding crusader types and mercenary or outright evil assholes; they are forced to compete for provisions and such, in ways that are meant to drive them into becoming Lawful Evil. The second prisoners are the soul shells of not-Lawful-Evil spirits who somehow ended up in Baator anyway; while they can't be tortured for divine energy (the Pact Primeval forbids it), Dispater does use them for ransom, usually by forcing loved ones who want them out of Hell to sign Faustian pacts in exchange for release.
Dispater isn't stupid, but he is insanely paranoid at this point. He's managed to survive dozens of schemes over the Nine Hells' long history, and while his position is perhaps the most secure of anyone in Baator, he's become increasingly twitchy about things. As a result, he's got half his servants spying on the other half. But, hey, he keeps the evil divine energy flowing into Asmodeus' coffers, he doesn't really upset the status quo hardly at all (which funnily enough doesn't endear him to his more ambitious peers), and his minions get to sink knives into each other occasionally to rise up in status a bit without pissing off the boss. If there's a place in Hell where things are "less shitty", this is probably the one.
Minauros
The Third Hell is a largely a cold, fetid swamp. The air is thick with fog and the stench of rot and decay. Stinking, oily rain and sleet fall from the leaden skies, which are periodically illuminated by streaks of green and purple lightning. Sharp ridges of obsidian thrust out irregularly from the muck along with small, muddy islands containing the twisted boles and skeletal limbs of lifeless trees. At the center of the great bog looms the layer's namesake: Minauros, the Sinking City.
The world-sized metropolis is built upon great stone plinths that thrust into the fathomless depths of the swamp below. Here petitioners are lashed to the columns, screaming and wailing with increasing panic as they sink helplessly, inch by inch, to drown in the brackish, frigid waters. The walls and avenues of the city proper remain in a constant state of decay as the city slowly, inexorably sinks into the ooze upon which it was built. Work details of petitioners constantly extract stone from the surrounding mire, using it to reinforce the city's support pillars in a futile attempt to delay its inevitable plunge into the muck. It is said that the current city is but the tip of an impossibly vast structure, as each layer is built upon the ruins of layers that have been reclaimed by the endlessly hungering mire.
The other large city on this layer is Jangling Hiter, the City of Chains, which is impossibly suspended from the upper level of Dis by enormous chains of impossible size and manufacture. The city looms like a ghost out of the fog and quagmire. Turrets, spires, and towers thrust skyward along with massive links of corroded metal, disappearing into the dense lower atmosphere. As its name implies, the entire city of Jangling Hiter is fashioned of chains. That's right: every dwelling, shop, tower, warehouse, abattoir, avenue, and alleyway is fashioned from links of metal which continually rust in the frequent rain, giving the entire city an acrid, metallic stench that barely overpowers the noisome stench of the swamp beneath. The chain devils (known as kytons) are among the Nine Hells' best torturers, able to flay soul shells with such precision and agony that even Asmodeus himself mostly farms out his torturing duties to these guys. For sport, they like to chase outsiders in their city and scare them into climbing up the chains holding up the city... causing them to shred themselves on the spikes and blades on those chains, cutting themselves to ribbons.
Mammon, the Lord of the Third layer, is both a greedy and miserly son of a bitch. He refuses to part with any of his precious gold pieces to find some way to keep his city from sinking (the weight of which may well be why it's sinking), and so he just forces everyone else to pay out of pocket to keep their own place aboveground. He's also one of the most treacherous shitbags in the Nine Hells, frequently plotting against everyone (including Asmodeus) and then willing to throw his good allies and friends directly under the bus when he's about to get fucked over for being caught. His current scheme is actually pretty smart and ambitious: he intends to destroy the economies of various safe and prosperous nations, in order to sow enough desperation and hatred that he gets a nice bumper crop of people signing over the souls in various pacts or other arrangements. Because he's got all his devils working hard on the Material Plane on this little project, his cities and strongholds are starting to become eerily depopulated. His minions, by comparison, pay handsomely for intelligence that advances this goal.
Hecate, the Greek goddess of magic, lives here in the realm of Aeaea. She has clockwork devices that are fed souls and provide magic energy (divine and arcane); she trades magic items she makes for the servants of Hell for more souls to keep her own works going. If she runs out of souls to use, no worry, she simply feeds her own fiendish servants into the machines.
Phlegethos
The fourth circle is the Hell that most resembles the stereotype of a fiery world of eternal damnation, filled with active volcanoes, rivers of liquid fire, molten rock, ash hills, smoking pits, unbearable heat, all wracked by tremors and earthquakes. Even the air seems aflame and thus Phlegethos is considered to be fire-dominant. The fires appear to be at least somewhat sapient, and lash out to burn vulnerable beings.
This layer is unique for nominally having two rulers. Lord Belial was formerly Archduke, and plotted during the political struggle called the Reckoning to take over Malbolge. He announced that Fierna was to rule Phlegethos in his name, but before he could move on Malbolge, Glasya stepped up and was given control of it by Asmodeus... forcing Belial and Fierna to rule jointly. Now, here's where it gets even more fucked up: supposedly, according to infernal rumors, Fierna and Belial are in some kind of depraved incestuous relationship. This rumor persists despite the fact they are also plotting and scheming against one another. Given that Belial is one of the smoothest-talking devils of all time, and that Fierna is apparently exquisitely beautiful even in human terms (and loves using seduction as a form of corruption), it's pretty much a 50/50 shot that the rumors are true.
It should be noted the Fierna has a very good friendship with Glasya, and is constructing her own network of allies and minions to stand on her own. This freaks out Belial, who is sure she'll depose him, but he actually doesn't want to destroy her, just... curb her ambition slightly. Fierna's palace is itself a den of vice which contains cells with her ex-lovers, which she alternately screws or skewers, as the mood takes her.
This layer has a rather interesting place for devils known as the Pit of Flame, a sea of boiling shit-water with columns of roaring flame. Even creatures immune to the searing flames here are still consumed with agony and suffering, so this is Hell's punishment place for devils. While most devils are terrified of the place, some see this place as a trial for themselves, a way to prove their loyalty or tenacity by voluntarily immersing themselves in the fires, so they pay for the privilege of torment.
But there is another, more compelling reason to visit this layer: Abriymoch, home of the Diabolical Courts, where not only devils can settle disputes between themselves, but mortals can get a second chance. Per the Pact Primeval, if a mortal feels that they were wrongly condemned by the terms of a Faustian pact, they can appeal their damnation to the Diabolical Courts for a reprieve (and while unlikely, it isn't entirely unheard of for someone to win their freedom via a sound argument). The city is also the home of Hell's armaments manufacturing, and is the one place in the cosmos where you can mine Baatorian green steel from the magma, which gives the city a certain economic edge.
The city is also the place where Hell's infamous soul-torturing devices, called Shrivers, are manufactured. Originally, they are used to flense souls and strip vital essence from them to make them into proto-lemures, or to make them into a kind of material used to manufacture certain items; the process is basically one of the most horrific and terrible things in the multiverse. However, clever mortals can use it as a magical location to give themselves some pretty sweet abilities for 99 days (which can be renewed with acts of corruption or obeisance, i.e. evil or lawful acts).
In a way, Phlegethos is the archtypical view of Hell: searing fires, insane levels of torture, and infernal courts where souls try - and mostly fail - to escape damnation.
Stygia
Stygia is fucking cold. Most of the layer is an icy, murky ocean fed by the River Styx. The history of its rulership is appropriately sordid. It was originally ruled by Geryon, who got caught up on the wrong side of one of the schemes to depose Asmodeus. His actual fate has been somewhat screwy: in 3rd edition, he was very nearly annihilated, ending up as a mere vestige for binders to summon, but as of 5th edition he is apparently not only back in a corporeal form, but he's actively fighting with Levistus, the current ruler.
Levistus himself was one of the "smooth talker" devils who thought he was such good shit that he propositioned Bensozia, the consort of Asmodeus (and mother to their daughter, Glasya). She basically rejected him, and in a pretty salty way because Levistus got so mad he actually killed her. Asmodeus decided that killing him in turn would rob him of some eternal torment, so he imprisoned Levistus in a block of unbreakable, unmeltable ice. When Geryon fucked up and got booted, Levistus was told he was in charge... but not free of his own ice prison. That prison still floats and drifts through the layer, and whenever Levistus tries to figure out some way to steer it (and thus gain some kind of mobility), it strangely doesn't work (almost entirely because Asmodeus likes fucking with Levistus and robbing him of even that tiny hope).
Tantlin, the City of Ice, is the big capital, and was in a state of anarchy for a while before Levistus put his boot down on it; he uses telepathy and other powers to work through his amnizu minions. There's also the Duelist's Chasm, where devils can settle their differences the old-fashioned way, to the amusement and betting of the crowds of other devils there. Devils normally can't just murder each other, so they have to get a license to fight like this, but they don't need any permission to fight non-devils.
Oh, there's one last place of high interest. See, Levistus' cultists have a very, very unique spell, bind to Hell, that enchants a weapon so that if you slay someone, their soul gets encased in ice and trapped in Stygia forever. A certain password will unlock them; if they can escape Baator, then they get to live (if they were mortal) or at least go home (if they were an outsider or something).
Other than that, it's the home of Set (which is fucking weird: it's on a glaciated wasteland covered with monuments and structures, with passages and catacombs through/under it all), and Sekolah (the sahuagin deity, looks like a giant white shark). Overall, if you don't have business here, it's kind of a lame place to stay.
Malbolge
Originally, Malbolge was an endless rocky slope made of craggy black stone and pits of fire. The air was hot and full of choking ash and vapors. There was no flat ground, so avalanches were common. Nobles dwelt in copper fortresses, the metal plating of which helped defend against the frequent rockslides. That, however, was before the gruesome fate of Malagard, the Hag Countess, reshaped the layer forever. Now, her exploded and grotesquely enlarged body is the layer, for all intents and purposes.
Malbolge has had the most known rulers of any layer of Baator. The first was Behrit, who was destroyed by Asmodeus for violating rules regarding devilish promotion. After him was Moloch, until his hag advisor Malagard tricked him into rebelling against Asmodeus. Moloch was deposed to Avernus (as a "rabble of devilkin", a pack of unique outcast devils) and Malagard was made ruler in his place. She didn't rule for long, though: Asmodeus made her body grow until it burst open in order to appoint his daughter Glasya as the new ruler, using Malagard's skull as her palace.
One special location is the Lakes of Bile, a nice little "factory" for an assortment of particularly nasty poisons that can be bought at a very high price. There's also the Tower of Pain, where the new Lord of the Sixth imprisons and tortures several enemies, such as the more loyal servants of Malagarde.
Maladomini
The seventh layer of Baator is made of ruins. Nothing exists here that hasn't been damaged, defaced, destroyed, or otherwise corrupted. Major tourist attractions, if you're crazy enough to try and tour the place, are Grenpoli, the city of diplomacy and treachery run by erinyes Mysdemn Wordtwister, and Malagard, the city of black spires from which Baalzebul rules over the plane. The Lord of the Flies is never satisfied with his domain, and always demands his cities torn down and reconstructed based on some tiny flaw. He wants nothing less than absolute perfection- even if he has to destroy his layer or all of Baator to achieve it.
Maladomini's theme seems to be how sin ruins good. Its ruler Baalzebul is a fallen archon named Triel whose pride drives him to continuously destroy his own realm because it isn't perfect enough, making the layer one gigantic ruin that will never be truly rebuilt. Triel went from being a beautiful angel to being corrupted into an obsidian-skinned, compound-eyed unique devil, and was later punished with the body of a giant, slimy slug for his part in the Reckoning. This is part of his image problem, and why he's constantly building and tearing shit down, trying to achieve a visual perfection that will never happen.
Cania
Much of Caina is a land of ice-covered boulders and mountains, ruins of stone and sprawling glaciers. The jagged mountains war with frigid glaciers, each grinding fiercely away at the other. Avalanches holding thousands of tons of snow rush down from the mountains regularly, crushing anyone slow or unlucky. A visitor will find Caina numbingly cold (-50°C). Without heat, most warm-blooded creatures can only survive for a few hours before suffering chills, later frostbite, and then death. Certainly any such creature that goes to sleep, falls unconscious, or is rendered immobile in the open without heat will die shortly.
Cania is among the most sparsely populated of the Hells, with a wary traveller standing a fair chance of avoiding notice. Were it not for the cold and the lack of food (some tales speak of remorhaz or glacier worms in Caina, but if any exist they must be very rare), Caina would offer intruders many inviting places of concealment there are many hidden valleys in the mountains, and countless ice caves. Beware though, the denizens of the plane are attracted to any fire, so in the unlikely even that any travellers are able to light one, it will likely be the last thing they ever do. The only parts of Cania that can be called even remotely hospitable is the area around the Castle of Mephistar, where Mephistopheles' experiments with hellfire have started warming the place.
Points of interest, other than Mephistar (the home of hellfire research) included Kintyre (an ancient city the Mephistopheles supposedly personally destroyed and/or transported to Hell), and Nebulat (where ice devils, displaced due to the new hellfire research, have retreated to try and figure out how to "reach" their poor archduke).
Nessus
If you make it this far, congratulations, you are truly fucked.
The final layer of Baator is a blasted oval plain, 2500 miles wide and 1100 miles tall, surrounded by that deadly void just outside of Baator itself. There's nothing on the surface of the plain: no trees, structures, no elevation, just flat, desolate wasteland. But across that plain are huge cracks, gorges, and canyons, some going miles down. It's in these crevasses where everyone/everything dwells. Because of this deceptive depth, the plain has a nigh-infinite amount of space (just down instead of across). Some of the canyons intersect and create labyrinths, others twist along their own solitary paths. Navigating is tricky, but maps provide a very slight help (though not much because of how hard it is to make a map when there are little or no landmarks to differentiate certain areas from others). Bridges across canyons are naturally guarded, and all of them are deeper than 200 feet (meaning if you fall and hit the bottom somewhere, you will be taking 20d6 damage).
And just what lives here? Asmodeus, Lord of the Ninth, King ov Hell (it's TRVE KVLT!). This guy is one of the true badasses in D&D, right up there with the Lady of Pain and a few other beings like Orcus. Asmodeus tricked all the Lawful gods, managed to conduct and eons-long war with the demons of the Abyss, and has done nothing but continue to add damned souls to his plane. He was kicked so hard out of the Upper Planes that his fall is said to have been the reason that Baator has nine levels, each broken off from the one above in the great fall. That fall also tore wounds in Asmodeus that still weep blood to this day... with each drop turning into a greater devil (usually a pit fiend with max hit points), and when they shed blood it becomes a lower-order devil in turn.
So yes, Nessus is fucking filled with Goddamned devils, all of them retardedly powerful and kept in Malsheem, which spreads for miles in four directions in the junction of two large canyons; the devils here are to be used to storm the Upper Planes, when Asmodeus' great plan comes to fruition. There's also Fortress Nessus, located at the bottom end of The Serpent's Coil (the torn path Asmodeus created when he hit Baator in his fall), a vast but seemingly empty structure, where Asmodeus supposedly stalks through the corridors, dreaming up new schemes.
But there's a site that is the real reason some dipshits actually try to make it this far: Tabjari, the fortress housing the Pact Primeval, which can grant enormous powers to subjugate Chaos if you have the balls to fight Asmodeus' favorite cultists for the right. (He can't put devils near the thing, because it's a perfect balance of Good and Evil, so it actually fucking demotes devils until they melt from lemures to Maggots and then into nothingness.)
Inhabitants
If it's not clear, devils (whose proper race/species name is "baatezu") basically run this joint. But there's old rumors and legends from the beginning of time that they aren't the original inhabitants. The stories go that there was a race of no-shit "baatorians" that once lived in the realm that would become the Nine Hells. It's not entirely clear what happened with them, but the indications are that Asmodeus put the boots to them in order to create a staging ground in the Lower Planes for his own invasions of the Abyss. Zargon the Returner from the book Elder Evils is a surviving Baatorian that Asmodeus couldn't kill due to Zargon's regeneration ability.
Aside from the devils, there are the various forms of Lawful Evil petitioners that wind up here when they shed their mortal coil. Most are just soul shells shoved into the mouth and innards of the infernal maggots that literally shit them back out as lemures, but there are other varieties. Certain souls become a kind of infernal specter in Dis (where their mental anguish provides a particularly renewable source of power for Hell), and the petitioners of the deities that live here get to go directly to those realms... but those deities sometimes sell them into bondage or worse, to get a little something back from the devils. There's a few other kinds of creature that roam the plane, such as hellcats (who aren't baatezu but certainly seem fairly "native" to the plane), hellhounds, and fiendish beings of every stripe.
Oddly, there's a lot of visitors to the plane, at least on the upper levels. Yugoloths are pretty common all around: they work not only as mercenaries, but they also have their infamous boatmen to move troops and goods up and down the Styx, as well as actual merchants who trade in souls, magic items, and other fun stuff (often acquired from the night hags who don't like going out this far). Certain kinds of beings like raksashas, efreet, and other Lawful Evil outsiders can also find themselves opportunities for business, work, or recreation (the last of which is better left to contemplate quietly). There are certainly mortal beings of all sorts who get down to one level or another, either making some desperate rescue of a lost body/soul, or hoping to strike a deal and slice off a little profit (either figuratively or literally, depending on how things play out).