Nine Hells of Baator

Rulers of Baator
Asmodeus is the ruler of all devils
 * 1) Avernus, ruled by Zariel
 * 2) Dis, ruled by Dispater
 * 3) Minauros, ruled by Mammon
 * 4) Phlegethos, ruled by Belial and Fierna
 * 5) Stygia, ruled by Levistus
 * 6) Malbolge, ruled by Glasya
 * 7) Maladomini, ruled by Baalzebul
 * 8) Cania, ruled by Mephistopheles
 * 9) Nessus, ruled by Asmodeus

River Styx
The River Styx also referred to the "River of Blood" travels through every layer of the Nine Hells of Baator. The river begins in Avernus at the Lake of Blood and continues until the final layer of Nessus, ending its journey at the doorstep of Malsheem. This is the most common way of travel through the Nine Hells, however the entrances and exits are heavily guarded by that layers minions. Those without permission usually end up being tortured along the river as an example to the rest.

Avernus, the First Layer of the Baator
It is a plane of darkness with rocky crags, deserts of ash and rivers of lava. Blood and viscera coat the surface of the realm, all of which is the remnants of past battles. Red and black clouds in the sky flicker with gouts of orange flame.

There are toothy mountains with sharp edges of crystalline rock. Many rocks in Avernus seem to have tormented faces etched into them.

Fireballs: The most prominent trait of Avernus are the fireballs that appear and explode. Fiery comets fall from the darkened sky and leave fuming impact craters behind.

Falling Souls: Sometimes, burning souls fall from the sky. The devils call them “crater makers.” These souls are actually lucky. They have the ability to get up and flee. It is possible for them to return to their old lives. It is said that this occurs when a devil contract is faulty.

Dis, the Second Layer of the Baator
Most of Dis is comprised of the massive city of Dis. Above it is a flat, smoky green sky in which lightning flickers.

Outside the city walls, the plains of Dis contain "wild" erinyes and hellcats. Mysterious pillars of stone move about the rolling hills.

The city of Dis is made of rusting iron, full of eyrie-riddled towers, zombies, garbage-choked streets alive with rot grubs and black puddings. A road of broken skulls leads to black walls. The skulls transform into spurs of hot iron but must be walked on to get inside.

The entrance is sudden. AS you approach, the walls of the city loom large and then you're instantly in a mazelike street. City details:
 * Population: 400,000.
 * The citizen include devils, damned, cambions, tieflings, planar travelers, spectres, corruption devils, abishai, lemures, spinagons, imps, rakshasas and erinyes.
 * The City of Dis is surrounded by a moat.
 * When inside, it looks infinitely large.
 * Abishai perch on the walls, keeping an eye on toiling petitioners.
 * The streets and walls are hot, and will do damage,
 * The walls and buildings have metallic rods that collect evil energy from suffering.
 * Much of the city is an uninhabited maze.
 * Some businesses in the city use flayed skins of low level devils as carpets and shades to keep the interiors cooler.
 * No two maps of Dis are the same. The city's configuration always changes.
 * Work is always being done on the streets. Slaves scream and burn as they labor.
 * Throughout the city,  there are iron statues of Dispater with glowing red eyes. They are scrying devices.
 * There are vents in walls of the city. In them, you can hear screams from prisons below.
 * The Hall of Infernal Might: A huge temple of Asmodeus in the heart of Dis. The damned worship him.

Minauros, the Third Layer of the Baator
This realm is an outland marsh of stinking earth covered with carrion and inches of water. Under the leaden sky are volcanic badlands, ash hills and lime-filled rifts. A never-ending cycle of polluted rain, sleet and hail pummels the realm. The sleet leaves an oily residue and the hail is jagged and sharp, flaying skin from bones.

Disease is rampant and there are underwater realms where giant shadow creatures roam. It is possible to fall through a slimy sluice and slide all the way down to the fourth layer, Phlegethos.

Phlegethos, the Fourth Layer of the Baator
Phlegethos is the classic vision of Hell, a place of fire and lava. It has the unique distinction of being a place where Asmodeus killed a god. It's a little confusing, but from what I can tell, in one edition it is said that the god was slain where Abriymoch stands. In another, it says the god died in or near the Lake of Fire. They could be two separate entities, not sure.

Phlegethos is full of volcanoes, fires, pits of smoking dung, hills of ash, and rivers of liquid fire. Above the black plains of cracked and cooling lava, the starless sky is lit by leaping flames.

The ground is so hot that mortals take fire damage each round here. Petitioners are frequently dropped into lava by devils.

Stygia, the Fifth Layer of the Baator
Stygia is an icy realm known for its great frozen sea and mammoth icebergs. The Styx winds through it but somehow does not mix with the waters. The realm is dimly lit by green-blue auroras of flickering frostfire and cold lightning that strikes most flying creatures.

Cold fires burn on rocky peaks after a lightning strike. Shooting stars of ice plunge into the swamp.

Avalanches and electrical storms are frequent, as are roaming glaciers controlled by Styx devils who delight in torturing lemures. The Styx devils believe that the laws of Hell do not apply to them, but they obey Levistus.

Malbolge, the Sixth Layer of the Baator
This realm has undergone a number of transformations linked to the changes in rulership. Originally, Moloch ruled Malbolge but was ousted by the Hag Countess. She was eventually transformed into an abomination that covered the layer, which is now ruled by Glasya. Glasya uses the giant-sized bones of the Hag Countess as buildings.

Apparently, creatures who drop to 0 hit points in Malbolge are dragged into the Hag Countess/ground by tendrils and devoured. Creatures known as kalabons are literally sentient chunks of her flesh that roam the land.

During the Hag Countess’s reign, Malbolge was on a tilt. Falling was a constant danger for travelers and boulders rolled around in magical patterns. Copperclad fortresses were built to withstand rolling boulders.

Maladomini, the Seventh Layer of the Baator
Maladomini is a realm full of fire pits and smoke under a blood-black sky. Bridges with diabolic faces cross rivers of molten fire, strip mines belch filth, slag heaps burn and filth litters the roads.

The realm is dotted with the ruins of cities and quarries where devils and slaves toil. In the ruins, petitioners hide along with beasts and devils who lost their territories. Some link to tunnels underground where there are creatures who lived in Baator before the time of the devils.

Maladomini is constantly being built and torn down due to Baalzebul’s perfectionism. Petitioners dig without tools, looking for stone and mineral. They build parts of a city and then are told to move on.

Cania, the Eighth Layer of the Baator
This lightless, frozen realm is incredibly cold, causing frostbite within hours. There are many corpses, frozen perfectly. Setting fires attracts ice devils.

Nessus, the Ninth Layer of the Baator
It is said that Nessus is so difficult to get to that anyone who does so is given the power of an ice devil or higher.

Nessus is flat and desolate. A network of gorges and canyons scar the realm, home to millions of devils. Any bridges are deadly unless the magic password is known.

Devils can't teleport in or out of this realm. Non-devils can use plane shift to do so.

Malsheem:
This is Asmodeus’s grand palace, a fey court of darksome beauty with a tangible aura of despair and filled with the sounds of weeping. It contains halls of gold-veined marble, walls covered in intricate gold leaf and furnishings of the finest quality.

In the center of the palace is a great magic gate that goes to any plane the user chooses. Any amount of creatures can pass through it. Asmodeus can't destroy or affect it. It is believed that in some mortal realm, there is a magic portal that connects to this gate. Details:
 * It is the largest citadel in the outer planes.
 * It is on the floor of the lowest rift..
 * It holds an army of millions of devils held in reserve for a cataclysmic battle that Asmodeus thinks will dwarf the Blood War.
 * When someone who is not lawful evil walks on the stone of this palace, smoke rises from their footsteps.
 * The guards and servants are pit fiends.