The Tarterian Depths of Carceri is a Neutral Evil Chaotic Outer Plane part of the Great Wheel cosmology of Planescape, and an abandoned plane in the Points of Light setting of Dungeons and Dragons. It is a prison plane filled with treachery, despair and hatred, where existence is harsh and escape is all but impossible. It is here traitors go to after they die, their treason and hatred lingering after their deaths; as such the people of Carceri are not to be trusted. Carceri is based on a combination of Tartarus from Greek mythology and Dante's Divine Comedy.
Layers of Carceri
Carceri has six layers, each nested inside one another. The layers take shape of spheres of finite size, each linked to others of its own size stretching into infinity. This means that the second and beyond planes are physically linked to one another despite being fully encased in its above layer(s)(maybe each shere is connected to another sphere via a tube that is fully encased in the corresponding tube for the above layer?). Despite this all layers have a red sky and light seems to come out of the orbs themselves, meaning that vision, while covered in something of a red tint, is normal on the plane.
The first layer of Carceri is reserved for politicians and traitors of nations. It is a realm of vast bogs and quicksand, the few dry parts quickly climbing into steep mountains hosting a race of malevolent titans. This layer also contains the River Styx, which flows freely throughout the layer and serves as a possible exit out of the plane. The layer also hosts vast numbers of mosquitoes and petitioners who annoy travelers. While not a prime vacation spot, there are much worse places to be found on Carceri.
More hospitable is the Bastion of Last Hope, a fortress located in a mountain ridge. Here live all sorts of spies, forgers, fences, assassins, plastic surgeons and all kinds of other people of ill repute who can, for a price, aid a traveler. But beware the nature of the plane of traitors when dealing with the people here.
Less treacherous but no less dangerous are the titans of Mount Orthrys, who were banished to Carceri long ago. Their leader, Chronus, can impart visitors with his wisdom. But because he and his people have been imprisoned on Carceri for such an untold amount of time they can get upset with visitors who can come and go at their own leisure.
The overgrown layer of Cathrys contains two kinds of terrain: highly acidic jungles whose mere air can dissolve a person within minutes and grasslands with razor-sharp leaves who can cut a person to pieces if they are not careful to avoid them. It is an overall unpleasant place where those who gave in to their base lusts and instincts are banished. The only reason one should travel to Cathrys is to visit the Apothecary of Sin, a place build above the forests of Cathrys where one can buy almost any kind of poison or acid from its apothecary, the glabrezu called Sinmaker. As long as he is paid he does not care what the customer buys, how much or what is done with his wares.
The third layer is nothing but sand. Minethys is an endless desert of sand and sandstorms, blown up with such force that it could strip the flesh off a man's bones in hours. As such, its petitioners (once those who did not use their wealth to help others when they could) are forced to wear heavy cloth garments to avoid being torn apart and live in hand-dug pits in the sand who constantly need to be dug out to avoid being swept up by a tornado. One of the orbs holds the Sand Tombs of Payaratheon, a city long since devoured by the sand. Those times it is revealed adventurers often try to explore it, discovering all kinds of petrified undead and sand gorgons who swim through the sand. Overall, Minethys is not a pleasant place and should be avoided.
The most inhospitable part of the fourth layer, Colothys, are its massive mountains and wide and deep canyons. Without some kind of flight or teleportation it is almost impossible to navigate given its narrow roads and few and far between bridges. Some trading routes exist, but they are long, perilous and slow to follow. Few things grow on this layer; those that do can be rather dangerous. An example of this is the Garden of Malice, which grows on a single orb. Despite looking beautiful with its many vines and flowers, the Garden is quite dangerous and will attempt to kill anything which comes too close. It is theorized that they're all under the control of a single organism which seeks to spread its seed throughout all layers of the plane. Colothys hosts those whose lies brought harm onto others.
Unlike the two layers before it, Porphatys is very wet. The fifth layer is completely covered in acidic water that can quickly dissolve unwary travelers. As such its petitioners (those who refused to aid others when the opportunity to do so presented itself) cling to the small sandbars across the layer, promising anything to anyone who can take them away. It is advised to not listen to them, since they will stab you in the back at the first chance they get. An interesting feature of the plane is the Ship of One Hundred, also called the White Caravel. It is a ghost ship of sorts that travels between the sandbars to pick up passengers along for its travels across the layer. The ship has no crew and is perfectly content with allowing its passengers along for its journey. What it does have is one hundred stone coffins (where it got its name from) on its lower deck. Nobody knows what they contain since every time one of the coffins is opened all passengers are devoured by some unknown disaster. As such people who know this story will leave the coffins alone and try to stop those who do try to open them.
The final layer of Carceri is something of a curiousity. Consisting of only one sphere, a ball of black ice with red streaks. It is lethally cold on the sphere and its petitioners are frozen in the ice up to their lips. The foremost landmark on this layer is the Necromanteion, the citadel of the death god Nerull. Here his priests perform necromantic rituals and experiments on hundreds of onyx altars in service of their dark lord, and houses many artifacts of the Reaper.
The main inhabitants of Carceri are the Demodands, who serve as the jailers of Carceri's petitioners/prisoners. Since the plane is occationally used as staging grounds or battlefields for the Blood War, all kinds of Fiends can be found here, including Tanar'ri, Baatezu and Yugoloth. Other evil Outsiders can be found here as well.