Designed as an eternal prison for the Nameless and its core minions at the end of the Dawn War, the Abyss is cut off from the direct influence of the Great Mechanism. With the events of the Cataclysm and the freeing of the Nameless and its minions (and its incorporation into reality), the Abyss is no longer a tightly guarded prison or a cyst in reality, but a haven for those who have made themselves unwelcome elsewhere. Without the circulation of anima from the Great Mechanism, abyssal denizens rely on other, less salutatory sources of power. Mostly, they consume souls. Some are willingly bargained away, others are sacrificed in blood-magic rituals.
Demenses and Habitations
Originally a light-less void where space, direction, and motion meant nothing, the new Abyss has been transformed by the Demon Princes and their followers. Landscapes and buildings were carved out of the void, created by force of will and buttressed with anima from mortal souls. Of the Five Princes, three are known to maintain demenses, one maintains a habitation, and The Jester's lair (if it has one) is unknown. These domains are usually in one of two forms--demenses or habitations.
Copies of the material plane or of astral landscapes, demenses are floating islands in the void. Maintaining such a pocket of normality in the hungry void of the Abyss is a sign of significant power. Three significant demenses are
Shachtat--Gallery of Corruption
Home to the Twisted One and his "artistic" distortions of form, this demense appears to be a vast ocean of dark water (although breathing is possible). Light only carries half as far as normal and radiant energies are muted. Strange slurping and gibbering sounds can be heard. The only lights are spotlights illuminating "artworks," living creature that have been twisted and warped into strange and horrific forms. These seem to be placed randomly around the lair. The Twisted One itself swims here, a corrupted kraken-like exile from Leviathan. His followers are also present, tending the "art."
Mechanical Elements: Motion is possible in three dimensions. The entire area counts as difficult terrain unless the creature has a swim speed. Resting in Shachtat is dangerous--at the end of a short or long rest a non-native creature must roll a Wisdom saving throw. The DC starts at 10 and increases by 5 for every subsequent rest attempt. On a failed save, the creature not only does not gain the benefit of the rest, but has disadvantage on all Intelligence or Wisdom based saving throws, attack rolls, and checks. This derangement persists unless removed by greater restoration or similar magic. The DC is reset only after spending a full week (8 days) outside of Shachtat.
Ternovna--Garden of Thorns
Ternovna appears to be a floating island in the midst of chaos. Approximately 10 miles in diameter (but irregularly shaped), this demense is dominated by the palace of Lloitira. Low and rambling with internal courtyards and gardens, Thorn-house appears inviting, almost sleepy. A formal hedge maze stretching over a third of the island; another section is covered in formal gardens. On closer inspection, all the plants are barbed, poisonous, and predatory. Unwary creatures get trapped and eaten by the maze (which shifts constantly) or by the hypnotic flowers. Deception is the order of the day. The palace itself echoes faintly with the sound of pleasure and pain. Implements of torture abound (and are wielded gleefully on and by the inhabitants). Illusion magic and shapeshifting are ubiquitous among the denizens of this place. Unlike the Twisted One (who collects only art and twists his servants to be more "useful), the Lady of Pain enjoys company and will often "hire" (knowingly or not) mortals to entertain her and her hedonistic court. She even pays them well, but very few escape without scars on body or soul from the experiences in that perverse place.
Physically the largest demense, Zubna is a vast jungle, dense with life. Unlike mortal jungles, everything here is savage and predatory. Unlike Ternovna, the predatory creatures here (including the plants) are hunters (some active, some ambush-style). Kill or be killed, eat or be eaten. Higher creatures (animals and humanoids alike) that die are reborn to start the cycle again unless unlucky enough to be killed and devoured by a soul-eater such as the Fang. Inhabited mostly by were-creatures and lovers of savagery and bloodshed, Zubna is dangerous place to visit. Stalking through this jungle is the fiercest predator of them all--the Red Fang himself. What civilization exists centers around the Grand Arena--a ramshackle pit where gladiatorial games take place constantly. All games are to the death (and usually end with the winner devouring the loser). Treasures can be won here, but the loser's soul itself is forfeit.
Unlike demenses, habitations make no attempt at appearing like landscapes or mortal places. They are generally rooms, connected to one another by portals, although some are tiny flat disks with "buildings" on them, open to the void, with atmosphere maintained by the will of the creator. They float in the void--breaking through the walls drops one into the unstructured abyss. What windows appear show false landscapes or nothing at all, as the owner wishes. These take less power to maintain. Of the Princes, only Oro-laen maintains a habitation. There are an uncountable number of such habitations. Some stand empty--their owners dispersed or consumed by one of the other denizens of the abyss or slain in combat with other powers. These empty habitations slowly dissolve into the void.
Dom-tizh--The Quiet House
Oro-laen has no patience for landscapes or games. His habitation is the Quiet House, a labyrinthine complex of chambers and tiny islands in the void, connected by gates. Experiment chambers, libraries, reagent storage, specimen habitats--all are present in abundance. The Black Lord himself maintains a tasteful abode, decorated in dark polished wood, stone floors that never seem cold, and steady source-less lighting. The Quiet House is home to a variety of denizens, all united in their love of the less-well-regarded branches of magical research. Blood sacrifice, necromancy, soul-alterations, studies of the Dark Beyond, all of these are conducted in the sanctuary environment of Dom-tizh. Travelers should be wary of making agreements or promises within the boundaries of this habitation--all such promises (no matter how seriously they were intended) are binding as if enforced by the geas spell (except that this effect has no maximum duration).
At the very heart of the abyss is Terminus, the Oblivion Gate. An inky-black spheroid approximately 10 miles in diameter, Terminus writhes like a living thing, sending out flare-like tentacles at whatever matter comes near. It sheds a form of anti-light, shrouding its surface in mystery. Everything else in the abyss revolves around Terminus, as if it were the anti-sun in this mockery of a solar system. If Terminus makes contact with matter, the matter is instantly and completely obliterated. This makes Terminus both greatly feared by the inhabitants and very useful as a garbage disposal. The origin and purpose of Terminus are unknown--some say it was created as the heart of the abyss and the rest simply accumulated around it. Others claim it to be the ultimate mercy--those Incarnations who no longer wished to exist could dive in there and find an end. Yet others say that one day, at the end of the universe, it will grow until it consumes all the planes and then the Dream will end. Observers have noted, however, that a single object seems to orbit very close to Terminus, well within the flare-tentacles' reach, yet undisturbed. No one fool-hardy enough to try to reach the object has ever reported back, so its nature remains a mystery. What is known is that the Oblivion Gate is the source of the jotnar and thus the ultimate source of demons and undead.