No plane inspires as much fear as the Abyss. This plane is the seat of ultimate chaos tainted by the darkest evil. It's said that Abyss has infinite layers, but "only" over 600 have been catalogued.

All the Abyssal layers have their own unique yet horrific environments. A planewalker will find lakes of caustic acid, clouds of noxious fumes, caverns of razorsharp spikes and landscapes of boiling rocks. Some layers have more hidden dangers: the layer that slowly changes a body's alignement towards chaos and evil, or the one that infects a sod with an incurable wasting plague.

The tanar'ri are the natives of this plane, they are 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. The tanar'ri have no sense of responsibility, honor or loyalty. Their behavior is always unpredictable. It's important to remember that what's deadly to many creatures may literally be harmless to a tanar'ri. In fact, a number of layers of the Abyss are so terrible that non-tanar'ri who enter them get put in the dead-book very very quickly.

The Abyss is the home of a number of other monsters, including eyewings, fetches, fire shadows, undead of all types, retrievers, bebiliths and others. It's also home to a number of powers, particularly those worshiped by chaotic monsters. Most of these powers have their own layer or layers directly under their control.

Vital to planar bashers 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, gates here lead down into the other layers of the Abyss. The tanar'ri lord Graz'zt rules three Abyssal layers, at least one of which is filled with trees with poisonous letal snakes for branches. However, Graz'zt is one of the few Abyssal lords who allows unhindered travel into and out of his realm. Chant's that a powerful mage died among those trees and that his decomposing bones hold a powerful artifact that renders a basher immune to the hostile environment of the Abyss itself.

Planewalkers should remember of a network of planars living in the Abyss that help those poor sods who end up here and want to leave. The Desderain, this is their name, use powerful spells of illusion and other magical aids to allow them to survive in the Abyss. The problem is finding these people when a body needs them. It mostly depends un luck, they obviously can't be everywhere or even on every layer. Not surprisingly, Desderain members are immediately slayed by tanar'ri once they're identified.