The Abyss (PWH)

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No plane, rightly or wrongly, inspires as much fear as the Abyss. This plane is the seat of ultimate chaos tainted by the darkest evil. Its virtually endless layers spiral deep down into the bowels of true vileness. No one knows whether the Abyss truly has infinite layers, but well over 600 have been catalogued.
All of the Abyssal layers have their own unique yet horrific environments. No unifying theme persists other than their harsh, inhospitable nature. A planewalker'll find lakes of caustic acid, clouds of noxious fumes, caverns of razorsharp spikes, and landscapes of boiling rocks—and those're some of the tamer dangers! Rusty, jagged pieces of metal cover one particularly harsh layer, some big enough to build a city upon, others so small that they blow in the driving winds to lodge in a hasher's flesh, mouth, and eyes. Other layers have more hidden dangers, such as the layer that slowly changes a body's alignment towards chaos and evil, or the one that infects a sod with an incurable wasting plague. Nevertheless, a few particular layers have interest to planewalkers, and it's best to head to those destinations directly; "exploring" the Abyss or wandering into uncharted regions is a good way to get killed fast (at best).
The Abyss is the home of the tanar'ri, creatures devoted to death and destruction. A tanar'ri on its home plane is likely to slay a planewalker or force him into serving in the Blood War or some other horrid duty. The best thing (for a traveler) about the tanar'ri is that they have no sense of responsibility, honor, or loyalty. They'll gladly accept a little garnish to look the other way—if the mood strikes them. Their behavior is always unpredictable. It's important to remember, too, that what's deadly to a human or a githzerai may literally be harmless to a tanar'ri. Just because they dwell safely in an area doesn't mean that it's safe for other planars. In fact, a number of layers of the Abyss are so terrible that nontanar'ri who enter them get put in the deadbook immediately. Therefore, even the toughest bloods need to be peery about shuffling about these layers.
The Abyss is the abode of a number of other monsters, including eyewings, fetches, fire shadows, undead of all types, retrievers, bebiliths, and worse. It's also home to a number of powers, particularly those worshiped by chaotic monsters like bugbears, beholders, lizard men, and drow elves. Most of these powers have their own layer or layers directly under their control.
Crucial to planar travelers is the uppermost layer of the Abyss, the Plain of Infinite Portals. Aside from being the easiest ingress and egress to the Astral, Carceri, and Pandemonium, crevice/gates here lead down into the other layers of the Abyss. Anyone passing through this layer should stop at Broken Reach, a fortress town run by a succubus (a type of female tanar'ri) named Red Shroud. Her rule is harsh and absolute, but it's not a bad place to find food, rest, and safety. Don't cross her, and she'll probably leave a basher unscathed—which is about the best a body can hope for in the Abyss.
The tanar'ri lord Graz'zt rules three Abyssal layers, at least one of which is filled with trees with poisonous snakes for branches. Despite the danger, though, Graz'zt is one of the few Abyssal lords who allows unhindered travel into and out of his realm. A planewalker willing to chance it can make a hefty bit of jink bringing goods to the market here. Besides, chant's that a powerful mage died among those viper trees, and that his decomposing bones still hold an artifact that renders a cutter immune to the harsh environs of the Abyss itself. 
There's also said to be a layer where time moves backward in regard to aging. This could be used to a body's advantage to restore lost youth. However, it is also said that the reverse aging moves at random speeds, and it's likely that a basher who stays too long will revert to an infant, or be aged · back out of existence altogether. 
Planewalkers should take note of a network of planars dwelling in the Abyss to help those poor sods who end up here (for whatever reason) and want to leave. These cutters, known as the Desderain, use spells of illusion and other magical aids to allow them to remain and survive in the Abyss. The trick is finding these people when a body needs them. It mostly depends on luck, since they obviously can't be everywhere, or even on every layer. This altruistic group of tanar'ri-haters knows that it's barmy to think that a body, or even a group, could significantly deal the tanar'ri a blow. There's just too blasted many of them. Instead, they have chosen to help the potential victims of the tanar'ri. Not surprisingly, Desderain members are immediately killed by tanar'ri once they're identified.
Tarsheva Recommends: Look for a vrock called Rcreen (not his true name, obviously) who dwells in the first layer. For the right garnish he'll show a body around and reveal some of the dark of the Abyss. Chant is, he's got a grudge of some type against his superiors and looks at this as a way to betray them.