Here is a brief review of some of the survivable layers of the Abyss.
One of the worst Abyssal realms is the 6th layer, the home of the Great Mother, ruler of the beholder pantheon. Every form of evil beholder and beholderkin dwells here, preying on one another and any tanar'ri foolish enough to wander into the crossfire.
The Phantom Plane is the 7th layer of the Abyss, though it has long been sealed against intruders. Weary of the Blood War and far from the front, Sess'inek is one of the few tanar'ri who spends little time concerned with the Blood War. The gates to this plane, more often called Kearakinin by the tanar'ri themselves, are all sealed: An extraordinary effort would be required merely to release the locks. Sess'inek is worshipped by lizard men, and his realm is rife with lizard kings, his servitors.
The 12th layer has been abandoned since the tanar'ri lords captured twelve devas and brought them here to be sacrificed. It's haunted by their tortured cries, which make conversation impossible. Twelvetrees is the site of much diabolical magic-working, for the fiends use the place where the devas died as a power focus to enchant engines of war and magical amulets capable of destroying the baatezu. The greatest project currently underway is a flying construction called the Ship of Chaos.
This was the site of a baatezu incursion into the Abyss, one of their greatest successes early in the Blood War. The 13th layer is now the realm of Beshaba, the Maid of Misfortune. Her realm contains both the black stags that are her totem as well as bad luck that haunts visitors to the realm. The reddish waters of the Blood Tor are also the home of Umberlee, the bitch goddess of the seas.
A bitterly cold plane of ice miles deep, the Iron Wastes are the province of frost giants who serve Kostchtchie, their tanar'ri lord. Devoid of most life, this 23rd layer of the Abyss is illuminated by a single distant light no brighter then the moon. Here Kostchtchie trained the first frost mages, who dwell in the Glacier Citadel, an ice fortress carved into the moving ice between two towering peaks at the height of the plane. Spring never comes to the Iron Wastes, and most of its inhabitants live deep underground in strongholds or caverns.
This infamous place is a convoluted plane that connects to the prime-material worlds where the spider-queen Lolth has worshipers. The plane folds in upon itself so that it resembles a great web. Four strands of tunnels wind through infinite mist, each strand looping in a circle and each strand passing (somehow) both over and under each of the others. Each strand is strung with gates into the planes where Lolth is strongest, as a string is strung with pearls. Lolth's palace is said to be a mobile iron fortress, perpetually crawling across her planar web. Her layer is the 66th layer of the Abyss.
The 74th layer, a realm of ever-shifting colors, is a radiant emerald jungle of heat, acid rain, and fermenting poisons. The jungle floor is difficult to find, as the forest has as many as seven canopies in places. The jungle is the home of Mershaulk, god of the yaun-ti, as well as many roving tribes of bar-lgura. Merrshaulk slumbers in a vast set of snake-infested pits and caverns, and is uninterested in being roused from his slumber by anything less than a direct threat. Most who intrude on his caverns are simply devoured; the rest survive only because of the power's all-consuming sloth. Ramenos, the god of the bullywugs, also sleeps in a realm within Smaragd, though his resting place is an enormous hollow tree. His tanar'ri followers call him the Sleeping God, and offer him sacrifices by placing them within his perpetually open mouth. Other thant the vipers, boas, pythons, venomous toads, and acidic rains, Smaragd is a relatively quiet and benign layer of the Abyss. Its dangers are passive rather than active.
This is a layer of briny water and rocky prominences used as rookeries by flying tanar'ri. The depths are ruled by kraken, ixixachitl, and their lord Demogorgon, whose stony home is guarded by the most powerful representatives of each of his servant races. Many wastrilith (water lords) make this layer their home as well. The area above water is only the slightest portion of his palace; most of it is underwater, in chill and darkened caverns that have never seen light. The tanar'ri lord hoards his strength, attacking in the sahuagin from time to time and only rarely exhausting his strength in the Blood War. This is the 88th layer of the Abyss.
A pungent expanse of baking, dry savanna and shallow swamplands is the home of Laogzed of the troglodytes, a gluttonous eater of carrion and spirits. Supposedly the offspring of Panzuriel and a reptilian tanar'ri mother, the stinking toad/lizard creature was quickly banished from Panzuriel's realm. Laogzed seems to have barely noticed; food simply became something more difficult to find. Laogzed serves other powers as a planar dumping ground, since he devours everything, regardless of its origin or foulness. Whatever he devours becomes part of the substance of the plane, so other tanar'ri often dispose of items permamently by placing them within a tasty morsel before that ultimate scavenger. Laogzed also devours herds of least tanar'ri, and those who disturb him with petty concerns (anything other than food). The Rotting Plain is the 181st layer of the Abyss.
The 222nd layer of the Abyss is more commonly referred to simply as the Slime Pits, a bubbling morass of oozing, fetid sludge. Its landscape is little more than the name implies, with vast expanses of caustic and unintelligent slime. Strange organic forms are believed to rise from the ocean of molds at the whim of Juiblex, who rules a portion of the plane. The remainder of the plane is the province of Zuggtmoy, the Abyssal Lady of Fungi, a dread and fell ruler of the Abyss. Together with Iuz the Old, himself a son of an Abyssal lord, she establishes the Temple of Elemental Evil and - for a time - wrought death, destruction, and great suffering. Then the forces of Good fought a campaign against her, and she was surprised and bound during the intaking of her Temple. The opposing clerics and mages confined her somewhere beneath the ground, reputedly with the aid of one or more deities. Until she is released, her realm will continue to slowly rot and crumble. Zuggtmoy's realm is filled with ascomids, basidironds, black pudding, brown mold, brown pudding, dun pudding, gray ooze, green slime, intellect devourers, ochre jelly, olive slime, phycomids, russet mold, ustilagor, violet fungi, yellow mold, and zygom.
Durao is an open layer that serves as the embarkation point for many tanar'ri hordes on their way to Gehenna. It consists largely of a set of rusting iron wharves and barracks built between a fetid section of swamps and the sluggish waters of the Styx. When troops march through the sprawling iron streets - which is most of the time - the entire layer seems to reverberate with the sound. The tanar'ri use enormous barges to ship their troops, though the marraenoloths are paid in small fortune to pilot these barges down the river. Other than rusty piers with access to the rest of the Lower Planes and the horrifically deformed creatures of the swampland (which Abyssal lords consider good hunting), the layer (number 274 by the Guvner's count) has nothing to recommend it.
The 303rd layer is a place where tanar'ri come to relax and smoke a pipe of dried flesh, dung, or whatever it is they smoke. The foul stuff clogs the air, making planars and pilgrims alike wheeze and ache with each breath. The stench is almost unbearable, though it doesn't bother the tanar'ri. Fires are constantly burning, pouring smoke and filth into the skies. Sulfanorum's fires are weak, though - just enough to light a pipe or start incense smoking.
A set of endlessly twisting tunnels dug from constantly-collapsing clay, earth, and stone. This 399th layer of the Abyss is the burrow of Urdlen, the mole-god of evil gnomes, who digs out from each collapse. The realm grinds up most visitors, and suffocates and poisons the rest. Its inhabitants are gnome petitioners, manes, purple worms, hezrou, and umber hulks. Oddly, many of the creatures of teh Worm Realm suffer from florid fungal and cancerous groths that rot the victim from within after 2-12 months; long-term visitors to the realm eventually contract this disease as well. Speculation among the Bargainers is that this disease is the result of a feud between Urdlen and Zuggtmoy, but this is impossible to confirm.
This is one of the few layers of teh Abyss that follows any sort of pattern or order. Throngs of petitioners and new arrivals are herded here from the entrances to the layer over treacherous terrain toward the layer's single visible feature, a great mountain; the gates exiting the plane are all within the central Mountain of Woe, but these gates are strictly guarded by the nalfeshnee, who are the primary occupants of teh 400th layer of the Abyss. They sit on mighty thrones of flame in the Mountain of Woe, or herd more creatures toward judgement. From atop their thrones, the nalfeshnee (also known as the lords of woe) cast judgment upon the mortal life of forces that pass into the Abyss, more dreadful than the judgment of the Guvners in Sigil. Rumors are whispered of the Pits of Despair, where the worst petitioners are sent.
This is the dismal and fetid 422nd layer of the Abyss, a layer of endless yellow forests and dun savannas where the gnoll god and his retinue hunt the lesser tanar'ri and any visitors. The grasses are edged as sharp as daggers, thornslingers and strangleweed are common, and the water is infested with parasites and diseases. Since tanar'ri are immune to poison, the waters themselves are venomous. Yeenoghu rules the layer from an enormous pile of cracked bones that looks like a white mountain visible from most points; those who have disobeyed or displeased him are added to it at regular intervals.
The 586th layer of the Abyss, this layer serves as nothing more than the prison of Diinkarazan, the mad god of the duerger. It's a swirling vortex of air and gas, with rings of whirling rocks flying about a central point. Diinkarazan is magically bound to a stone throne at the center of the storm. The plane constantly distorts space. Cursed and banished by Ilsensine, the power of the illithids, the Mad God cannot be freed by anything less than a greater power, and he is permanently insane and tormented by illusions of th things he most fears (terrible monsters, Ilsensine itself. drwoning in water or lava, and the like). The chant is that the high-up stops raving once every 50 years, but he's obsessed with revenge and kills visitors even when he's sane, as like as not. The prison is always torn between slipping over into Carceri and slipping into Pandemonium; the balance of prison and madness keeps it in the Abyss.
Kali rules a place of perpetual blood sacrifice and suffering. The plane is home to bloodthirsty, four-armed xorn, eyewings, fetch, fireshadows, and hordes of fanatical petitioners. The petitioners constantly slaughter one another and are reborn to kill again. Likewise, the layer's caverns are constantly being created and destroyed, and only the goddess knows what tunnels will seal up next. Visitors are grist for the altars. The Caverns have no regular gates - they're hidden near the most powerful proxies. These gates can only be activated by slaying the proxy. The Caverns are catalogued as the 643rd layer of the Abyss, though Guvners assume that many travelers found their way into this layer before anyone found a way to tell of it.