The Nine Hells

The Nine Hells, Hells, or Baator is an outer plane that is in between lawful neutrality and evil.
The Hells are ruled by Archdevils and devils that live on each of the 9 layers. There is a very clear and defined system of order, class, and law on this plane even amongst such beings. Each layer is controlled by one Archdevil and the terrains and environment vary drastically. It is unknown if the Devils that control it shape the layer or the layer shapes the devil. Additionally, the layers get smaller the farther down you go, as entering the Hells the traditional way would put you at the top. The one thing that is consistent however, is that it is unhospitable and deadly for a visitor. Even so, the strict chain of command follows even the most powerful of lords all the way down to Asmodeus.
Avernous
The first layer of the Hells is called Avernous. The landscape here a desolate wasteland of sharp rock and obsidian jutting out ready to cut the inattentive. This land has seen countless battle and war resulting in blast holes of varying size and depth. Dotted throughout were low mountains and volcanoes that streamed rivers of lava and fire throughout, the air was poisonous with smoke and ash, storms of fire streaked through the dark red sky and exploded often near living beings, corpses and bones littered every mile. There was so much bloodshed that rivers were formed, every drop would eventually flow into them. Any who wished to enter any layer of the Hells had to first come through here. Asmodeus wanted any army to have to conquer layer by layer in order to get to him first. Thus, this layer had the most devils present at any given time, ready to fight another war when needed. The ruler of Avernus was titled, Lord of the First, and currently that Lord is Zariel. She was once an angelic Solar of Mount Celestia but fell through her grace and became an Archdevil. Her sole purpose is still pure on paper however as she deals with the armies of demon kind pouring out of the abyss from the river Styx. She is the frontline and always in the need for more troops. She has her subordinates and lesser devils go to the material plane and to make contracts and give mortals the abilities they need to fight so that when they die, they come to Avernous and serve her army. Through this layer one could find the Bronze citadel which housed Zariel and her army of thousands, a several mile wide road that has been carved by the armies marching through the eras, and pillar of skulls which is one of the first things a visitor will see as it is several miles high as a trophy and warning to any would be invading demon army.
Dis
The second layer of the Hells is called Dis. Dis was notably flat compared to most and despite being potentially infinite, the Archdevil in charge decided to keep in finite within a ring of mountains that very few are able to pass through alive. Across 90% of its entirity, until the cirlce of mountains, is a city of the same name. This city is made entirely out of iron and burned red hot, every wall, and road stone burns to the touch and creates plumes of smoke that leaves the city in a grayed hue. Through different sections of the city all flowing towards the center palace are black rivers that stink of decay. Upon entering Dis from Avernous, you would appear on the outskirts on a road of skulls leading to the city. From there, you can see the Iron Tower as you can see it from anywhere on this layer, the home of Lord of the Second, Dispator. Dispator is an Archdevil and master strategist with an extreme intellect. His Iron Tower serves multiple purposes, the first is his impenetrable home line with an uncountable number of floors layered with traps, devils, and all matter of death, as well as to house the expansive knowledge that he's collected. The deals he makes with devil and human alike, are those for knowledge. He'll find souls that are unable to seek the paths in life that they desire simply because they're uneducated. Also on this layer, you will find the Garden of Delights, a establishment that offers the most decadent of pleasure to all the senses.
Minauros
The third layer of Baator is Minauros. The entire layer is a stinking and acidic swamp. Other than the various devils lurking around other dangers included an acidic rain and sleet that left an oily surface on the thick murky water and often followed by a razor hail. The main civilization is simply, the City of Minauros controlled by Lord of the Third Mammon. This city was constantly sinking into the muck below it, Mammon needed a constant workforce of lesser devils going out and fetching more stone and obsidian to reinforce and build it back up again. Mammon is greed and selfish incarnate, amassing possibly the largest amount of wealth in existence. His power and influence come from a duality of wealth, he can hire mercenaries, give loans, buy honor and service, but the more he spends the less power he has. This has given him an absolute sense of influence but also an irrational fear to protect his treasure.
Phlegethos
The fourth layer is Phlegethos. A climate so unbearably hot it seems that the air itself is on fire. The ground would erupt in earthquakes that spewed out more flames from the fissures, each fire seemed to have a sense of sentience as they seem to jump out and towards other beings or anything that could help it grow and spread. This plane was most notable for its court that was stationed there and lead by the two Lords of the Fourth, Fierna and Belial. Belial was more concerned with the contracts and laws of the Hells and would see most of the proceedings of the court. Every dispute among devils was handled through here, most often being settled in favor of whatever devil had the most power but when a pit fiend or higher devil brought shame upon themselves, it was here that they were demoted and brought down in power. Fierna is a much more cunning and fiendish devil and handles the contract side of the two. Her contracts specialize in making people able to speak the way that people who don't play D&D think persuasion works.
Stygia
The fifth layer is Stygia. Named after the river Styx that criss crosses and flows freely through it. Stygia is a frozen ocean of saltwater that's shallowest point is several miles deep. Most of the surface however is covered in a thick layer of ice occasionally broken up by the cruising icebergs and glaciers. The sky was stuck in a perpetual gray twilight and stuck in a permanent lightning storm making flying difficult for most creatures. Some of these glaciers were big and still enough to hold cities on top of them, traveling from location to location is made possible only by the Styx which keeps its oilier water flowing and never freezes. Lord of the Fifth is Levistus who currently resides in the center of a large iceberg as a prisoner, punishment for attempting to overthrow Asmodeus, this iceberg prison was also made impenetrable and allowing Levistus to never die which is both a blessing and curse as he can never be killed but will never die and always be a prisoner. Levistus still has power of the layer and can communicate orders with his devil through telepathy which can have a range of 10 miles.
Malbolge
The sixth layer is Malbolge. The entirety of Malbolge is a layer wide prison made for devils by Asmodeus. The outside is covered in decayed forests, swamps, and a desert which can be looked at as the prison courtyards. There you will find Hellwasps, Pit fiends, hellhounds, and other manner of things meant to keep people in. Not even devils dared to visit this layer because if you were seen, it was assumed you were a prisoner and would be killed or suffer an eternity in its cages. The Lord of the Sixth is named Glasya, the daughter of Asmodeus. As the Princess of Hell, she is extremely powerful but is as much a prisoner as she is a warden. She is considered the most unpredictable of devils and takes the liking to appear in a succubi like form.
Maladomini
The Seventh layer is Maladomini. Much like the rest, the landscape on this layer has a red sky, rocky terrain, and horrible air quality. Scattered throughout are abandoned and ruined cities that made great hideouts for criminals, lesser devils, and runaway slaves. The wasteland around the cities are littered with surface mines, deforested areas, clogged canals and rivers blocked by sludge, and masses of corpses that have long since decayed. Lord of the Seventh is currently Baalzabul, a fallen Archon with an obsession for perfection. Believing he was so perfect he attempted to overthrow Asmodeus, as punishment, the dark lord transformed and twisted his soul into that of a slug. Eventually Baalzabul was able to transform himself into that of a insect like humanoid once again. Baalzabul is in a constant flux of commanding his subordinates to build him a new and more perfect city. He would live in it until he found himself dissatisfied, order everyone to abandon it, and make a new one.
Cania
The eighth layer is Cania. This layer was packed with frozen danger. Much like in Stygia there are frozen oceans, massive glaciers, and icebergs. Unlike Stygia, the icebergs here move at a constant speed equivalent to a fit man running. The glaciers often crash into each other causing large waves of freezing water or nearby mountains covered in snow to tremble and create a potentially deadly avalanche. Accumulated snow often builds up over ravines and crevices that would cause a being to fall helplessly and be buried in snow. The air itself is so cold that it could freeze a being solid in minutes if they did not have magical means of protection, corpses fully preserved can often be found of devils that were foolish enough to travel here. Lord of the Eighth was Mephistophalese, a brilliant and master of the arcane arts. His prime focus is on experimentation and perfection of spells, transformations, divinations, and all other manners of arcane manipulation. His contracts are littered with loopholes that will grant a being power or ability but will then force the being to work for him or be subject to his often cruel experiments.
Nessus
The final layer is called Nessus. If you are able to make it all the way through the 8 layers and manage to find and pass through the portal to the ninth, you will be transported to a stone bridge inside of large cavern that has an egg like shape. several thousand miles in diameter along the horizontal plane and nearly ten thousand miles tall. The walls of this cavern while not smooth, are blank, barren, with very little detail other than the cracks in the rock that have various streams of lava trickling out of them. In one of the nooks drips ever so slowly the Styx as it forms its own small lake at the bottom on top of the mass of lava. In the center of this cavern is a thin stone spire with the outer planes largest citadel Malsheem in the direct center of the cavern as the spire continues to top. Most that would be invited would have to be by Asmodeus himself, considering that if he wanted you there, he would bring you immediately to the citadel. As such, cavern is guarded by all manner of devil both big and small and kill on sight any that are uninvited.