Baator

This is it: the Big One, the Nine Hells, the Pit of Darkness, the Stinking Mire, whatever. It's got more names than there are prime-material worlds. It's the plane of the most dangerous evil there is: the baatezu, fiends of unsurpassed power. These monstrous beings are absolute masters of their plane. Among their kind exists a rigid hierarchy of domination that defines all life on Baator.
The most vile petitioners consigned to this plane are transformed into larvae - grub-things found throughout the Lower Planes. All petitioners endlessly suffer the tortures of the baatezu, for the wretches can never truly die by torment. Their burnt, broken, and bleeding carcasses reform within a day and the entire process begins again. The petitioners endure this in the hope that someday they will advance and torture others.

Geography

Each of the nine Hells was unique and usually mirrored the malevolent characteristics of its ruler, or perhaps the archdevils were shaped by the domains they schemed to control, no one can be certain. the domains of the archdevils were described as territories (large, but finite) or circles. The relationship between layers and circles is not fully known.
One cant understand the Nine Hells without understanding the layers that make it up the layers are: Avernus the blasted, Dis the iron city, Minauros the stinking mire, Phlegethos of the flame, Stygia the great sea,Malbolge the crushing, Maladomini of the ruins, Glassia the glacier, and Nesslis the deepest pit.
Avernus: This is a rocky wasteland with a dark red and starless sky, though the landscape is lit by spheres that weave and flare into fiery explosions. The blasted plain is scoured by savage legions, ready to repel invaders. This host is led by Bel, a pit fiend, eagerly earning honors from the infernal archduke of the plane. Tiamat, Queen of Darkness, guards the entrance to the next layer. The River Styx flows through this layer.
Dis: The smoking black walls of the iron city Dis, which is also the name of this layer, rise into the ash green sky, and narrow streets run as far as can be seen. Condemned petitioners toil at meaningless labor, one team tearing a building down as fast as another erects it, and all work is done without tools. The fiery iron burns the flesh, and the streets echo with cries of agony. The archduke of this layer rules the Iron City from a tower of lead and stone.
MINAUROS: This is the layer of greed. Foul rain, oily sleet, and razor-sharp hail sweep across the layer. All is bog, save the ridges of volcanic glass that slither through the landscape. The only city is Minauros the Sinking, built of black stone, forever settling into the mire. Fiends drive petitioners onto the bottomless bog to find ever rarer stone to bolster the city's structures. Decayed bodies gurgle to the surface, filling the air with disease.
PHLEGETHOS. Here is the legendary realm of fire, filled with volcanos and rivers of liquid fire. It's virtually the same as being on the plane of Fire. The only city is Abriymoch, built in the caldera of an almost extinct volcano.
STYGIA. This is the realm of ice, a great frozen sea, although there's open water where the River Styx flows. Small plants grow here, creating an icy swamp. Lightning scours the sky. On a great ice floe stands the ice-crusted city of Tantlin.
MALBOLGE. This layer is a great rockfall, immense in size. With no clear path, travelers must scale a constant parade of building-sized boulders or wind through the dark tunnels they form. The sky burns with clouds of red steam. There is no single city on this layer, but a series of copper-clad fortresses among the scree
MALADOMINI. This is the plain of ruins. Under the blood-black sky, petitioners quarry, carve, and build new cities for the archduke of this layer. The land, once fertile, is scarred by mine pits, slag heaps, brackish canals, and half-destroyed ruins. Cities are built upon cities, but only one, the newest, Malagard, stands above the surface. Beneath, the ruins are dungeons so extensive that even the lesser fiends fear what lurks in them.
CANIA. Worse than Stygia, this layer is solid ice. Exposure here is equal to that on the plane of Ice. Huge glaciers grind at the jagged mountains that hold them. The archduke of this layer rules from the citadel of Mephistar, overlooking the glacier Nargus.
NESSLIS. This is the deepest pit of Baator, a plain shattered by rifts deeper than the deepest ocean trench. At the lowest depth sits the palace of Baator's current overlord, a citadel even greater in scope than Khin-Oin on the Gray Waste. It's unmapped and undescribed, which suits its fiendish master just fine. The citadel stands on the shore of a lake of fluid ice that feeds the River Lethe. This layer is a land of extremes — the coldest freeze, the hottest fires, the steepest cliffs, etc.

Fauna & Flora

Baatezu, hellcat, hell hound, hordling, imp, larva, and simpathetic call Baator home.
Within the strict chain of command, the greater baatezu constantly jockey for power, each attempting to overthrow its rivals and become the supreme master of Baator. Takhisis of the DRAGONLANCE saga resides on this plane, although the Clueless of Krynn think she lives in the Abyss.

Natural Resources

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).

Alternative Name(s)

the Nine Hells, the Pit of Darkness, the Stinking Mire

Type

Dimensional plane