The plane known as the Nine Hells was mentioned for the first time by name in the article "Planes: The Concepts of Spatial, Temporal and Physical Relationships in D&D", in The Dragon #8, released July 1977. In the article Gary Gygax describes the plane as one of the "Typical lower planes". The plane was mentioned again in an appendix of the known planes of existence in the original (1st edition) AD&D Players Handbook, published in June 1978, where it was described as "The Nine Hells of absolute lawful evil".
The article "The Politics of Hell" by Alexander Von Thorn in Dragon #28 (1979) describes how Hell was originally ruled by Satan, who was usurped by Baalzebul, who was in turn overthrown by Asmodeus.
The article "The Possessors" by Arn Ashleigh Parker in Dragon #42 (October 1980) follows up on this article by explaining that Selm, a lieutenant of Satan, was made the Prince of Possessors, and remained in that position throughout these various changes in the administration in Hell.
The principal inhabitants of Baator are the devils, fiendish creatures of pure lawful evil; the most populous variety of devils are the baatezu, a race which effectively rules the plane. The devils are in a constant conflict known as the Blood War with the chaotic evil demons. The ultimate rulers of Baator are the Lords of the Nine, also called archdevils or Archdukes; each one rules absolutely over one of the layers. The current political climate of Baator was determined by a civil war known as the Reckoning of Hell.
Besides the devils, Baator is home to evil deities such as Tiamat and Kurtulmak, as well as hell hounds, fire giants, rakshasas, and other evil creatures. A few mortals live in well-defended fortresses in Baator.
Baator is also populated by several types of petitioners, the most common being soul shells. These are ghost-like forms which can be molded by the devils into increasingly horrific and agonized forms; ultimately, their destruction results in their essence merging with that of Baator. Especially evil petitioners become lemures, mindless blobs of molten flesh who serve as shock troops in the Blood War and basic servants to other devils.
In earlier Planescape supplements, another type of creature, the nupperibo, is said to inhabit Baator as a remnant of an ancient race that existed long before the Baatezu. Nupperibos grow naturally from larvae present on the plane if left to develop. Since the arrival of the Baatezu, the larvae have been molded by the devils into lemures in an effort to prevent large numbers of nupperibo developing.
In the 4th edition cosmology, the Nine Hells are still the haunt of the devils. However, Asmodeus himself is the only evil deity to dwell in Baator (Tiamat shares the Dominion of Tytherion with Zehir), and the nupperibo and other ancient Baatorians do not exist, as the devils have inhabited Baator since before their fall.
In its original conception, Baator is a spatially infinite plane, consisting of nine layers or sub-planes, arranged in descending order like a dark pit, the inversion of Mount Celestia's ascending mountain.
In the standard D&D cosmology, Baator’s first layer, Avernus, shares borders with the neighbouring planes of the Infernal Battlefield of Acheron and the Bleak Eternity of Gehenna; travel is possible between Baator and these planes at certain locations. Baator's portals are huge hoops of reddish light guarded by abishai and amnizu.
In the Forgotten Realms cosmology, Baator links via the Astral Plane, to Toril, the Prime Material Plane. A number of portals directly connect it with the Barrens of Doom and Despair, Clangor, and Blood Rift.
The layers of Baator are configured as a descending cone, with each layer successively lower than the one before it, as opposed to the Upper Planes of good alignment, where one goes higher to enter deeper into the plane.
In the 4th edition, Baator's overall structure is finite. The new Baator is shaped like a planet, with each layer taking the form of a continent-sized cavern deeper within the planet's crust than the previous one. Avernus is the "surface" of this "planet", while Nessus clings to the underside of Cania, overlooking the planet's fiery core. In addition to being a realm of torment, Baator doubles as the Astral Dominion of Asmodeus, god of tyranny; like all Astral Dominions, Baator drifts on the endless currents of the Astral Sea as opposed to having a fixed position on the now-defunct Great Wheel.
Baator has nine layers:
The first layer, Avernus, is a vast charred wasteland of rubble over which the iron towers of the Dukes of Hell stand. Legions of devils march across the plains in continual readiness for the next battle of the Blood War. A red light suffuses the sky and huge fireballs fly across the layer, randomly exploding wherever they hit. Beneath the blood-red sky balls of fire burst unpredictably. They are said to be embodiments of the rage of the imprisoned Lord of the First, Zariel.
The River Styx flows through this layer, as does the appropriately named River of Blood. From the gate in Ribcage the nearest site is the ruined city of Darkspine, where refugees desperately try to avoid the baatezu thought police while trying to do enough good to shift their town back to the Outlands.
The Bronze Citadel is a huge fortress city, housing massive numbers of devil troops and war machines. It is constantly being added to in the form of new fortifications against attacks. Lord Bel, formerly a powerful pit fiend before his ascension to noble status, is the current ruler of Avernus, having betrayed the former lord, Zariel, to obtain the position. He resides in a personal fortress at the center of the Bronze Citadel.
The largest series of astral conduits on Baator lead to the Maggot Pit, a sea of squirming infernal larvae, beyond which the Dragonspawn Pits of Azharul sprawl in a cluster of tall hills and mountains. Tiamat, the five-headed deity of evil dragons, stands watch over the way to the next layer with her draconic and abishai servitors. Nearby exists the Pillar of Skulls, a hideous landmark of trophy-skulls of those killed in the Blood War that reaches a height of more than a mile close to the entrance to the second layer.
A number of godly realms exist on Avernus, including Draukari, realm of kobold deity Kurtulmak, goblin deity Bargrivyek's deceptively titled realm of The Peacable Lands, and Abthalom, the Nether Reaches, realm of dragon-queen goddess Takhisis. Vast legions of abishai serve the Queen of Darkness, milling through the Rallying Grounds and near the Temple of Takhisis and the Tower of High Sorcery. The towns in Draukari, hidden in the hills, mountains, and deep forests of the realm, are Frekstavik, Nibellin, and Snjarll. Kurtulmak's realm is often raided by goblins serving Bargrivyek.
A number of unique devils cast from the hierarchy of the Hells also dwell on Avernus. Known as the "rabble of devilkin", these exiles number roughly 50 former consorts, dukes, vassals, and other unique devils who lost favor with their masters. To prevent them from being summoned, they have been stripped of their names.
The second layer, Dis, is both a layer and a burning city of iron known as the Iron City of Dis. The walls of the buildings of the city radiate extreme heat, as do the stones of the streets; more than brief skin contact results in severe burns. The Archduke Dispater rules this layer from the Iron Tower, an impregnable fortress that reaches far into the sky and can be seen everywhere on Dis (literally; by looking to any direction in the skyline one can see the Iron Tower).
A notable feature of Dis is God Street, a place filled with buildings that are actually the godly realms of various lawful evil deities that have not earned enough worshippers to earn a realm on another layer of Baator or another plane entirely. It is suggested that homebrewed (DM created) lawful evil deities get a realm on God Street.
Although the Iron City of Dis has a wall, the transition from plain to the city itself is imperceptible. One moment a traveler is passing an iron spur, the next the traveler is in the middle of the city with the mighty iron walls surrounding everything that can be seen. It is thus much easier to enter the Iron City than it is to leave it; to leave, one must pass through the heavily guarded city gates (unless the traveler has means of crossing planes of existence).
The iron walls and streets of the city smoke with intense heat, burning those who touch them without protection. Abishai, erinyes, spinagons, and lemures are common - so are soul shells, petitioners who retain their humanoid forms and memories, the better for Archduke Dispater to torment them. Bezekira, kocrachons, rakshasas, and hamatulas are also found thronging the streets of Dis.
The most prominent building in the Iron City is Dispater's tower of iron and lead, a skyscraping edifice that constantly changes shape. The streets, too, are constantly shifting, constantly being rebuilt by the tormented petitioners, confusing and sometimes choked with garbage.
Despite all this, Dis is the most populous and wealthy city in Baator, filled with planewalkers of all types who come to sample the city's terrifying delights.
The realm of Druaga, the Refuge of the Fallen, exists on the plains far outside the city walls. Dis is largely flat, with a sky of smoky green occasionally lit up by lightning. Black, stagnant rivers criss-cross its plains. Monolithic spurs of blackened, unworked iron - natural formations, as much as anything in Baator is natural - thrust out of the plains, growing more frequent as a traveler approaches the Iron City itself.
The third layer, Minauros, is an endless bog of vile pollution. The weather on Minauros consists of acidic rain, flesh-slicing hail, and harsh winds. Most of the layer is a vast dismal marsh of foul rotting soil, littered with carrion and pools of water. Through murky fog, one often encounters numerous carcasses soaking up the filthy rain. Another horrid feature of these lands are "cells." These are large shallow pits, filled to a depth of two or three feet with water. Chains and manacles of brass and iron are attached to huge stones laying at the bottom of the cells, where intruders and others captured by Mammon's barbed devil sentinels will be kept. The chained prisoners are forced to stand or sit in the chill, fetid water until they die from exposure or starvation, unless they are taken away for torture or interrogation first. Fortunate prisoners escape, though occasionally the barbed devils will allow a prisoner to escape in order to hunt it for sport.
The huge stone city of Minauros the Sinking is located here, so called because the weight of the city causes it to continually slip beneath the slimy waters; only the endless efforts of thousands of slaves prevents it from doing so. The city sits in a marshy bowl in the center of these volcanic lands, and is constructed of a black stone gleaned from another plane, perhaps a world on the Prime Material, and rests on mighty pillars that eternally sink into the foul bog that fills the layer. Below the city are said to be the ruins of a town that once resided in the Outlands, a town filled with riches beyond dreaming. Hanging in the sky over the stink of Minauros can be found the kyton city of Jangling Hiter, City of Chains, suspended by chains suspended from who knows what.
Mammon the Viscount is the ruler of Minauros; he resembles a long serpent with a human torso. He rules from the centre of the city, within a huge mausoleum-like structure made of gold.
Aeaea, the secondary realm of the goddess Hecate, is on Minauros, as is the realm of the Blood Queen of the aboleths.
Phlegethos is a fiery wasteland filled with active volcanoes, hills of ash, gouting fires, streams of magma, pits of smoking excrement, and burning sand beneath a screaming rain of magical fire flakes. Lady Fierna and Archduke Belial are the lords of this layer. Few can exist here for long without suitable protection from the extreme heat. The fortress capital city of Abriymoch is built on the lip of a volcano and composed of hardened magma in the caldera of a volcano and is somewhat more bearable than the rest of Phlegethos. It has little tolerance for outsiders, but important allies of Fierna, the Lord of the Fourth, have their places in its society. The pit fiend Gazra, commander of the hamatulas, dwells here in a palace of crystal. Fierna and Belial reside in a palace of pure obsidian high up on one side of the city.
The ground of Phlegethos is always uncomfortably hot. This, as well as patrols of barbed devils, keep most intruders constantly on the move. Tremors are common, and it is not unusual for the ground to violently erupt or fissures to open without notice. The sky of Phlegethos is a dark, starless void, but constantly leaping flames provide weird illumination to the harsh landscape.
Rivers of liquid fire feed and emerge from at least two massive lakes where the flames burn brightest. This fiery "water" is inhabited by numerous wandering salamanders that were brought to the layer in days long past, to be bound in servitude to the master of the plane. Given that salamanders are not prone to taking orders, this didn't work out, and most were slain by Belial's servants and vassals. Those few who escaped remain bound to Phlegethos by Belial's magic, and continue to survive by avoiding large groups of baatezu and snatching lone individuals who venture too close to one of the fiery rivers.
The Pit of Flame is a boiling, fiery sea of refuse empowered by the primal energy of Baator. It is a place of both promotion and punishment for baatezu across the nine circles. Thousands of osyluths watch it to ensure that it is not abused.
Most of Phlegethos is inhabited by barbed devils, though other baatezu live there as well.
In the fiery plains beyond Abriymoch, the Sumerian goddess Inanna lives in a realm called the Jealous Heart.
Stygia, the fifth layer is a freezing layer of cold and ice dominated by a murky ocean fed directly by the River Styx. The surface of the ice is covered with chilly marshes. The dark sky is constantly filled with lightning storms, and the ice may be lit by weird cold flames. Tantlin, the City of Ice, is built on a huge ice floe with a large harbor to the river Styx, and is ruled over by a huge pit fiend. The lack of any kind of law enforcement leads itself to gangs controlling much of the city.
Prince Levistus rules over this layer, frozen in a giant iceberg floating in the harbor as punishment for betraying Asmodeus. For many centuries, his realm was given over to the fiend Geryon. Geryon was recently deposed, and Levistus was reinstated, though he remains in his iceberg.
Godly realms on this layer include Sheyruushk, realm of sahuagin deity Sekolah, and Ankhwugaht, realm of Set.
Baatezu of this layer include amnizu, osyluths, and cornugons. In the water are giant octopuses, sharks, squids, whales, sahuagins, and scyllans.
Some also still serve Geryon, who lives now in his Citadel Coldsteel. There he broods with his minotaur worshippers and thinks of vengeance, but mostly he still does the bidding of the Dark Lord of Nessus, who holds the key to his restoration. Doing his bidding is the wolf-headed duke Amon, though some claim that Amon is not as loyal as he seems, and serves Levistus in certain matters as well.
Beneath the ice is Sheyruushk, the realm of the sahuagin deity, Sekolah. On another ice floe is the realm called the Steadfast Chill, where Kriesha (a goddess of Cerilia) dwells.
Ankhwugat is the realm of Set. Set is the dark master of the desert, lord of serpents, jackals, assassins, and creatures of the wild. Though dark waters flow underneath his realm, its surface is hot and dry as the sands of Pelion, and those familiar with the realm of Nephythys there will see Ankhwugat as a twisted reflection of that one. When Set and Nephythys divorced, Set went to Stygia to forget his pain. Perhaps once all Stygia was like Ankhwugat, and perhaps, if Set has his way, it will be again. Some claim that Set is mad and wishes to transform the layer or even turn his realm into a new outer plane set between Baator and Acheron. Others say he only wishes to reunite with his estranged wife Nephythys, and that the mad ones are the baatezu who try to earn his favor. The common chant says that Set and Levistus are headed toward a showdown. The will of a Lord of the Nine is supposed to be absolute in his layer, but Set has the good will of Ra and Levistus is imprisoned in ice.
Malbolge was originally an endless rocky slope where rockslides and avalanches of boulders were common. Numerous copper fortresses provided some refuge from the avalanches. Once ruled by Moloch, he was deposed during the Reckoning of Hell and replaced by The Hag Countess Malagard. Malagard has since died, her body expanding and imploding to the point that the entire layer is literally her remains, and Glasya, daughter of Asmodeus, currently rules Malboge. Now, the realm is alive, being formed from the flesh and bones of the Hag Countess.
As "The Crushing Lands", Malbolge was a place of craggy, black stone and ash filled with stinking vapors, smoke, fire pits, and huge caves and caverns in which ancient Baatorians lurked. The air was hot and choking, and the whole layer existed at a steep tilt so that no flat terrain exists. Falling was a constant danger there, as were rockslides and avalanches.
The Sixth has had more known lords than any other layer of Hell. The earliest known ruler of Malbolge, the archdevil Beherit (along with his consort, Batna), was destroyed by Asmodeus for violating rules regarding the promotion of devils. Subsequently, the layer was ruled by Baalzebul through his viceroy, Moloch. When Moloch defied Asmodeus after the Reckoning of Hell, the former was exiled to Avernus, while rulership of the Sixth was passed on to Moloch's leman, Malagard the Hag Countess. Malagard's reign was short-lived, however, as she was deposed by Glasya, Asmodeus's daughter and current Lord of the Sixth. As of Fiendish Codex II, Glasya used the Hag Countess's body to reform Malbolge. Thus, the Sixth plane of Baator is in fact little more than the Hag Countess's innards and bones. The Countess is technically dead, though her life essence still remains on Malbolge. The tunnel made from the remains of her throat still expands and contracts, as if something immense was breathing shallowly.
Malbolge is now a noisy place populated by tormented lemures, cornugons, and the occasional spinagon. Large numbers of lesser devils on this layer are missing all or part of at least one limb, or have some other sort of disfigurement or infirmity—a testament to the days when Baalzebul and Moloch ruled here, both of whom delighted in the torture of their subjects.
The nobles of this layer once dwelled in copper fortresses, whose metal plating helps ward off the worst of the falling stones. The Hag Countess once traveled between these fortresses in disguise, testing the gentility of her subjects; she despised mindless brutality, and destroyed those who committed it.
After the Hag Countess' demise, those few nobles were not purged by Glaysa for treachery now live with the Princess of Hell in her palace, a grotesque, five-story building formed from Malagard's skull.
The 4th edition of Dungeons & Dragons retains much of Malbolge's history. Malbolge's residents include barbed devils, cambions, chain devils, legion devil veterans, and succubi. The hellwasp devils - mezzodemons that were corrupted by Glasya during the Blood War - keep their hives in her Garden of Delights.
Maladomini is a hell of ruins, the seventh layer of Baator. Everything natural on the layer has been defaced, destroyed, or stripped away, replaced with strip mines, quarries, poorly maintained roads and bridges, rivers of molten lava, slag heaps, wasted cities, and polluted canals, filled with stinking vapors, earth tremors, fire pits, and mines. Petitioners and other refugees hide in the ruins. Underground, the ancient diabolic predecessors to the baatezu still lurk.
The archduke Baalzebul, the Lord of the Flies, rules this layer. He is never satisfied with his cities, always demanding that they be destroyed and rebuilt based on tiny, unnoticeable flaws. Nothing less than perfection will do for him, even if he has to destroy his entire layer - indeed, all of Baator - to achieve it.
The major intact cities are Grenpoli, the city of treachery and diplomacy run by the erinyes noble Mysdemn Wordtwister, a fortress called the Relentless, and Malagard, a city of black spires from which Baalzebul himself rules.
Cania (or Caina), the Eighth Hell, is an unimaginably cold wasteland, where few living things can survive more than a few moments in an unsheltered area. Huge fast moving glaciers crash around the layer sending snow and ice up in the sky. It is dominated by intense, freezing cold, and is extremely hostile toward those who are unwelcome on the layer, which includes nearly every being that enters. It is the home of the gelugon baatezu.
The layer is ruled over by Mephistopheles, from his great citadel of ice Mephistar. It sits upon a giant glacier called Nargus, the movement of which is controlled by Mephistopheles himself. Inside Mephistar huge heated baths and fire warm the citadel, providing quite a comfortable environment.
Many things lie hidden under the ice of Cania: vast lost cities, frozen armies of creatures. The primary portal to access Nessus is through a gaping hole guarded by 9,999 gelugons. At the bottom of the dizzyingly deep pit is an icy body of water and 1,001 fathoms deep in that is a silvery portal to Malsheem.
Nessus is the ninth and deepest layer of Baator. It is a place of pits and ravines of virtually endless depths. The great citadel of Malsheem lies immediately below the portal between Cania and Nessus and is the largest fortress in all of the Outer Planes. Here, Asmodeus rules over the entire plane, and thus the entire race of devils.
The surface of Nessus is a plain shattered by rifts deeper than the deepest ocean trench. Many of the ravines and canyons here reach thousands of miles into undifferentiated, dead stone. Most of the trenches seem natural, but some appear as if they were cut or blasted into the land. Rumor has it that an offshoot of the River Styx flows here and there, dropping into trenches and trickling its way across the layer. Few know how to reach this tributary, if it exists at all.
A rift of incredible depth and width lies immediately below the layer boundary between Cania and Nessus. Malsheem, the Citadel of Hell, rises in its dark, elegant, fiendish beauty from the trench. From here, the Overlord of Hell, Asmodeus, rules the plane of Baator.
In 4th edition, Nessus's appearance has been completely revamped: it is now a spherical cavern surrounding the core of Baator. The devils of the ninth dwell in iron citadels that protrude from the crevasses and stalactites formed by Nesus's "floor". Malsheem itself protrudes directly from a massive volcanic crater. Nessus is inhabited by the brazen devils (the guards of Malsheem), firebred hell hounds (Asmodeus's pets), storm devils, legion devils, pit fiends (the aristocracy of the Hells), and war devils.
The topography of Baator is broadly derived from the Hell described in Dante Alighieri's The Divine Comedy, although the arrangement and names of the layers of Baator differ greatly from the circles of Hell described by Dante. The Outer Plane of Mount Celestia is likewise loosely inspired by Dante's depiction of Heaven and Purgatory.