Carceri

From Manuel of the Planes (3e):

It is a plane of exile.

It is the prison plane of the multiverse.

It is where the overthrown plot their return.

Carceri seems the least overtly dangerous of the lower planes, but that first impression quickly disappears. Acid seas and sulfurous atmospheres may be rare on this plane, and there are no areas of biting cold or infernos of raging heat. The danger of Carceri is a subtler thing.

The plane is a place of darkness and despair, of passions and poisons, and of kingdom-shattering betrayals. On Carceri, hatreds run like a deep, slow-moving river. And there's no telling what the flood of treachery is going to consume next. It is said that a prisoner on Carceri may only escape when she has become stronger than whatever imprisoned her there. That's a difficult task on a plane whose very nature breeds despair, betrayal, and self-hatred.

Unlike most inhabitants of Carceri, the deity Nerull makes his home on Carceri willfully, not because of exile.

Carceri consists of six layers. Each layer has a series of orbs like tiny planets, in a row. A gulf of air separates each orb from the next. On a particular layer, little distinguishes one orb from the next, and it's possible that the number of orb-like planets on each layer is infinite.

Carceri is called the sixfold realm because it has six layers nested within each other like little wooden dolls. On each layer, a strand of small planets stretches in two directions toward infinity.

Many of Carceri's layers are battle-scarred and wasted, the legacy of the Blood War. While much of Carceri remains oblivious to the war raging across the lower planes, portions of Carceri are used as staging grounds and even battlefields.

Vision is normal on Carceri. Unlike on the Material Plane, natural light seems to seep upward from each orb, bathing everything in a reddish light. Hearing is normal on Carceri.