Carceri
The prison of the multiverse, the pit from which none escape. The Tartarian Depths of Carceri are mysterious to most citizens of the Crossroad Realms, since those who go in and have a good look around don't tend to come back out. The exceptions have been gods, demigods, and dubious legends.
Carceri is where the vilest villains in the multiverse are sentenced to spend their eternity. It is the lockbox into which horrors that cannot be killed are flung. It is the reason a person travelling the Realms by magic must always, always, double-check their tuning fork.
Geography
The layers of Carceri are arrayed like a string of beads. At the centre of each of these twenty-five planetoids lies a portal to the next Tartarian layer. The innermost layers have the smallest planetoids with the greatest distance between them, while the first layer had planetoids each about the size of Primus, with about 100 miles between them.
These planetoids do not rotate unless the will of a greater deity wishes it, fixed stoically in their places. Being a pocket plane, the sun of the Crossroad Realms does not shine on Carceri, but each orb gives off a dull red glow like the phosphorescence of a fire beetle.
Partly due to natural gaps in the planetoids' substance, and partly due to mass tunnelling by the prisoners, each of Carceri's planetoids in each of its layers contains an expansive underworld. At the centre of each planetoid lies a portal to the next-deepest layer, with the centre of Agathys containing an exit to the Crossroad Realms. As no records exist of anyone reaching this portal or escaping through it, however, this last remains speculation. However, owing to this legend, much of the tunnels in Carceri are man-made attempts to reach this sure-fire exit.
Localized Phenomena
Besides blocking all manner of transport and communication spells from reaching the plane, Carceri's Realm Magic also prohibits any egress from within, even via the River Styx. Legends speak of secret ways out (usually guarded by fearsome creatures), but scholars are adamant that these are Macguffins for fairy tale heroes, and not reality.
Carceri's Realm Magic also extends to transitory planes such as the Ethereal layer and the Echoes. While Carceri is said by some to have Fey and Shadow copies, these copies, like the original, are bound and travel outside them to other Realms is impossible. Approaching Carceri via the Ethereal Realm, one simply passes through the space as if the world were folded around it, similar to the wards over Baator.
Climate
Carceri's weather betrays the cosmic tug of Chaos on this insular plane. Seasons and weather occur seemingly at random, and what little plant life grows on the planetoids rapidly matures and withers over a matter of hours.
Fauna & Flora
All manner of creatures can be found in Carceri, either as petitioners or prisoners.
The souls of creatures who committed some great act of betrayal - usually against a god - are sometimes sent here to serve their afterlives in eternal imprisonment. These petitioners have physical forms and all the memories, albeit hazy ones, of their lives.
Some prisoners were jailed in Carceri for reason, others are unfortunate accidents. Travellers who spelled their portal gate wrong, or fell asleep drifting down the River Styx, or through some other mishap ended up in every wanderer's nightmare.
Many such prisoners have been in Carceri long enough to establish permanent populations, though they are not, strictly speaking, indigenous to the plane. [Titans] are not native to Othrys, but cannot be found outside it, and many monstrous species such as trolls and all forms of undead can be found in the Prison Plane, as well.
Native creatures include the Gehereleth wardens and their Abhoroloth servants, as well—if you believe the legend—as the Prisoner under Agathys.
History
Carceri was apparently constructed as a prison plane shortly after the birth of the Abyss, with the idea to contain the infection within it. When the futility of this became apparent, it was repurposed to contain other kinds of evils.
This tale is purported mainly by Devils, and is thus considered dubious, but has been corroborated by some gods, as well.
Others whisper a different story; that Carceri predates the Abyss, and even many of the gods. That the Prisoner sealed under the deepest layer is the reason the plane was constructed, and that its builders sacrificed themselves to create the eternal seal of its Realm Magic. These legends warn, that should the ice of Agathys melt and the locks of the Prison Plane break, no god or hero will be able to save the world from what comes out.
Alternative Name(s)
the Tartarian Depths of Carceri
Type
Dimensional plane
Location under
Inhabiting Species