The Tarterian Depths of Carceri, also known as Tarterus,is an Outer Plane in the Great Wheel cosmology model representing alignments between chaotic evil and neutral evil, situated between Hades and the Abyss.

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.

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). 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.

The Bastion of Last Hope is a rare outpost in the cursed swamps of Orthrys, the first layer of Carceri. Once a prison, it is now known across the multiverse for being a safe port of call for Anarchists, rebels, and criminals of all stripes, and its market boasts nearly anything a spy or criminal would need, from complex forgeries to magical potions or tools, to permanent alterations for disguises.

The Bastion is under the control of members of the Revolutionary League, Anarchists who believe that organized power is inherently corrupt. True to their creed, they place no one individual in charge, instead rotating leadership roles (or “facilitators” as they prefer to call them) among their membership. The Anarchists of the Bastion are divided into four main collectives, each one denoted by a color. The Blue collective is the militant wing, believing force of arms is the only way to uproot the corrupt power structures of the planes. The Red collective are skilled infiltrators, spies, and rogues. The Green collective believes in the power of words and diplomacy to sway reasonable beings to the Anarchist cause, and the Yellow collective believes that the mercantile system and the merchant class are the greatest threat to the search for truth, and that all resources should be shared equally by need.

Visitors are usually welcomed into the Bastion, but are restricted only to certain areas. The Main Hall is where the merchants and dealers set up shop, and where visitors can purchase thieving gear and disguise kits and the like. The Mess Hall serves as a tavern, selling food and drinks. The cellblocks have been turned into living quarters, divided by collective with the exception of one block that is reserved for visitors.

There are few rules in the Bastion. Visitors are expected to stick the Main Hall, Mess Hall, and living quarters, and to keep their weapons and spells to themselves. Aside from that, they're free to stay as long as they wish. Beds and the stables are free, but food and anything else costs, so the impoverished rarely stay longer than they can afford. Most visitors to the Bastion also remember that they're on a plane full of traitors, and stay only as long as they need to inside the Bastion's walls.

The party visited the Bastion on their journey to the Red Pit, and on their return. (2,4)

Zaphon is an Aasimar, a half-celestial human, living in the Bastion of Last Hope in Carceri. Little is known about him, and he rarely volunteers information about himself. But he is nonetheless a minor legend for being the only known inmate of the the Red Pit to every successfully engineer an escape.

He stays in his own private chambers in the Bastion, enjoying the protection of one of the Anarchist collectives that runs the place. His room is filled with maps, drawings, plans, and logs of various prisons, and he seems to take both pride and pleasure in his ability to break into or out of secure places.

Zaphon gave the Party information, and acted as their guide, on their journey to the Red Pit. (2,3,4)

Located in Orthyrys, the first layer of Carceri. A path of white marble steps, known as the Thousand Marble Steps of the Titans, meanders around the slope of this mountain, leading up to the top, where the tip of the mountain touches the tip of another mountain stretching from a neighboring orb. Where the tops of the mountains touch, a speck of white is visible; the Palace of the Titans. The path is blocked at its base by a large black stone fortress, the home of the Horned Knights.

The Planeswalkers scaled the slope further along the base of the mountain until reaching the path in order to avoid the knights, fighting a Giant Spider, and then a Harpy on their way to the Palace. (5, 6)

The Horned Knights have their fortress located at the base of Mount Orthrys. Martigan first encountered them when he escaped the Baatezu War Camp he was contracted to serve at with another mercenary, Charlie. The two of them were captured on the way to the Palace of the Titans. (However, Martigan does not remember these events). Martigan has a brand of the Red Knight's emblem on his hand, likely as a result of that event.

Although the Party attempted to avoid the Knight's Fortress when they were headed to the Palace, Charlie led them into an ambush with the Knights, which they managed to overcome. Martigan spared Charlie's life in exchange for information on a man in Sigil who could help him escape his contract.(5,6,7)

Located at the peak of Mounth Orthrys, on the first layer of Carceri, the Palace of the Titans is the home of Cronus, King of the Titans, and many others of the same race of being who were exiled there by the Greek Pantheon for unknown crimes that predate the existence of mortal races.

The Palace is built of cracked and crumbled white marble, cast red in the hellish light of the plane, and consists of a number of buildings and sites, including an amphitheater, a village for the Titans, a farm of grain and livestock, and a small tent city built by and for mortal visitors seeking shelter from the plane or the wisdom of the Titan King.

The Party visited the Palace in order to enter a Portal that would lead them to the fourth layer of Carceri. (5,6,7)

The tent city exists at the whim of the Titans, who tolerate the lesser beings in their midst only so long as their fickle tempers allow it. One day an angry Titan will take some offense at the assembled beings and vent their frustration, but until then the place stands. The tent city is nominally led by a woman named Althea Damaskaros, who operates a hostel and kitchen for travelers in the largest tent at the center of the sprawl. She claims to be a priestess of the Titans, worshiping Cronos as a god. Rumor has it she was once a priestess of the Greek Powers before some betrayal that drove her into the arms of their exiled enemies. Regardless, her worship pleases and amuses the fickle Titans, and goes a long way towards ensuring the tent city's continued existence. (6,7)

Within the Palace proper, Cronos the Titan King sits on his throne, bound by adamantium chains forged with divine power. Unable to leave his Palace, Cronos sits and broods, swinging between bitter melancholy to blinding rage at a moment's notice. Many come to the Palace to seek his advice, for he is almost as old as all creation itself, and knows many things. But visitors always run the risk of triggering the Titan King's unpredictable rage, and many visitors who came seeking knowledge found only death instead.(7)

(See also: Red Pit Info)

The Red Pit is a prison, itself located inside the Prison Plane of Carceri. It is owned and operated by the Mercykillers, one of Sigil's Factions dedicated to Justice and Punishment. Information gathered in Sigil suggests it is an oubliette for prisoners whose crimes have personally offended the Mercykillers and their leader, Factol Nilesia.

As with all things on Carceri, the Red Pit is more than it appears, and functions as a prison on more than one level. The Mercykiller guards posted at the Red Pit are also prisoners of a sort themselves, banished to this remote outpost either for gross incompetence or for displeasing their higher-ups. They know they will never be relieved of duty or transferred out, and so they take out their frustrations on the prisoners.

Lady Raven DuBois reportedly has connections to the Red Pit, despite not being a member of the Mercykiller Faction, and uses it as an oubliette for her own enemies. Cal's father is known to be one such prisoner, and has been rotting away in the Red Pit for several decades.

The prison consists of a fortress atop a rocky spire in the middle of a foul lake, with four watchtowers. A guardhouse at the edge of the lake contains four guards watching a bridge leading to the entrance. The fortress itself contains a staging area, barracks for the guards, the shrine to Hextor, and the Warden's quarters. The prison levels are underground, dug into the rock of the spite. A shaft leads straight down, opening in the void below the layer and down to the orb of Cathrys, the second layer of the plane.

Each level of the prison contains a number of cells, an interrogation room, and a guardroom with two guards. Each guardroom contains only the keys for that level's cells and logbooks of only that level's prisoners. The stairs up and down through the prison layers wend through each guard room, making it impossible to travel down without encountering each set of guards.

Another point of interest is the lair of Sheol, a juvenile Tartarian Dragon a short distance away from the Red Pit. Rumor has it that Warden Crackjaw once tried to negotiate with Sheol to become allies and work together against the petitioners and criminals of the plane, but the negotiations fell through and the two have been bitter enemies ever since.

Their enmity proved pivotal in engineering a jailbreak from the Pit, when Sheol was tricked into believing Warden Crackjaw was planning a preemptive attack on her and she opted to attack first instead. The results and aftermath of the battle are unknown, but it's safe to assume the fortress and the garrisoned guards have both suffered from the battle. (2,3,4)

Thanks to maps and information provided by Zaphon, it is known that the Red Pit's Warden is a Deathknight named “Crackjaw”, who rules the place with an iron fist. Reputed to be a legendary swordsman in life who attracted the attention of Nerull in death, he has thrown his lot in with the Mercykillers out of a desire to punish the wretches of the plane he has grown to despise.

30 living guards are garrisoned at the Red Pit, most of them the lazy, undisciplined cast offs of other Mercykiller fortresses who wanted to be rid of them. A chaplain runs a shrine to Hextor, and a retinue of walking dead attend to Warden Crackjaw(3).

Sheol is a Juvenile Tartarian Dragon, a Planar dragon native to Carceri, who makes her lair in an abandoned fortress near the Red Pit, on Orthrys, the first layer of Carceri.

While most Tartarian dragons are polite and well-mannered, in spite of their unquestionably evil souls, Sheol's youthful exuberance makes her more outspokenly vain, arrogant, and pugnacious than the rest of her kin. These flaws often overshadow her natural intelligence and awareness, driving her to rash or impulsive decisions.

Sheol has a bitter enmity with Warden Crackjaw of the Red Pit. Stories say that he once attempted to negotiate an alliance with the dragon to join forces against the criminals and petitioners of Carceri, but Crackjaw's failure to show proper deference and respect to Sheol made the deal fall through, and the two have been bitter enemies ever since. The two stayed at arm's length from each other, Crackjaw because he didn't wish to risk the loss of men and resources in killing a dragon, and Sheol because she knew Crackjaw was at least a match for her power, and she might not live through the encounter.

The uneasy truce was broken when the Planeswalkers tricked Sheol into attacking the Red Pit as a cover for their jailbreak of Cal's father. The fate of Sheol, Crackjaw, or the Red Pit itself remains a mystery. (3, 4)

Colothys, the fourth layer of Carceri, is a place of preposterous high mountains and deep canyons. Mountains on Colothys soared hundreds of miles high and have slopes that make climbing outright impossible. The mountains support trees, but these don't serve as good handholds for climbers. There are nonetheless some ways to make traveling through Colothys easier. Natural plateaus serve as rest places and artificial rope bridges between canyon walls (hard to use due to the wind on the layer) and the rare ledges, about 1-foot-wide (0.3 meter) carved into the sides of mountains, aid travel and served as trade routes, but are fought over by the inhabitants.

Petitioners, in Colothys's case, the spirits of liars and cheaters—and giants live on Colothys too. Denizens organize themselves in small villages on the canyon walls. They fight each other over necessities like food and water, the aforementioned ledges, and other supplies, but also out of a simple desire to kill. Denizens are easy to provoke into lethal fights. Legions of demons and daemons make lairs in the canyon walls, as well.

Gallowshome is built around a portal to Sigil inside a cave. Inside the cave is a five-mile expanse, the floor (half a mile below the entry ledge) covered with titanic bones, and the ceiling hangs with titanic bodies dangling from ropes, their throats all cut. Maggots infest the bodies and drop from them to an unknown destiny; the maggots, which attack any who come near, are the souls of sinners and are supposedly keeping the giants dead.

The Party travelled to this region from a gate in the Palace of the Titans on Orthrys, and had to fight their way down a narrow pathway to the bottom, to the Portal and the Riddling Reaver's Tomb.(7,8)

Located at the bottom of Gallowshome, in the skull of a three-eyed creature. It was said to house an artifact, the Keyring of Olidammara. However, after nagivating it's traps, the Planeswalkers found neither the Keyring nor the Reaver's remains; they did find treasure a note from the Reaver.9