The Dark Plane is a plane created by Saetan Dagon Samael before his death and Emmeline. For a plane built by mortals, it is extremely large - spanning dozens of miles.
Features of the Plane
All gravity in the plane is normal - save for in the Cave of Mysteries, where gravity is subjective.
The Dark Plane is timeless, though healing and poison still functions normally.
The Dark Plane has no geometric shape (the map provided shows the plane's borders). At the border, the ground abruptly breaks off into empty twilight. Trying to travel into this twilight will randomly send you to a different plane.
There are only three ways to get into the plane. The first lies in Sam Holloway, who - through the absorbed power of Saetan's gloves - can teleport there or create portals to there at will. The second lies in Emmeline's cloak, which acts as a permanent portal linked to Portal I, Portal II, and a portal in Cryshal-Tirith. The third lies in any duplicate of Cryshal-Tirith Saetan raises. On the third floor of the tower is a portal linking all incarnations of Cryshal-Tirith together.
All of the bright areas on the map experience weather and day/night cycles normally. All of the shadowy areas are in ever-night.
Attempting to teleport forces the wearer to make a will saving throw DC 30 or be affected with Divert Teleport - the location being the Everdark Dungeons.
See Crenshinibon for information on Cryshal-Tirith. Cryshal-Tirith contains Saetan's vast library - including a complete set of Nether Scrolls that Blair created post-deification to absolve herself of her binding to him. Currently, Cryshal-Tirith is sealed shut and is impossible to enter even through magical means. Only one weilding Crenshinibon or the Ebon Jewel could enter - and Crenshinibon is sealed within the libary itself. Only Sam Holloway can currently access Cryshal-Tirith, and she seems unaware that the strucutre is not a plane in itself but part of a much larger plane.
Blackcity is home to roughly 2,000 of Saetan's followers, though the rest of his followers are able to access it should they need to. Blackcity is a sanctuary, a resting place for those who follow The Black Prince. There, they do not age and can rest until they leave the plane - re-entering the Material Plane at the same time they left. There are numerous shops and services located throughout the city.
Soulmine is a place when unwanted prisoners of The Black Prince are harvested, either for liquid pain or souls. It also doubles as a torchure chamber, and a prison for those who aren't enough of a threat to make it into the Everdark Dungeons. Soulmine conains a Demonic Graft Machine, a Pain Pit, and a Greater Dark Altar Stone. All of these items were created by Saetan, and imbued with the power of Bane. They count as relics.
Skeletown is the home of the intelligent undead under Rose's leadership.
Former victims of Soulmine are buried in the graveyard one they have been finished with. The Graveyard is also a frequent haunt of Rose' mindless undead.
Whisperwood is an extremely dangerous place. Some of the more dangerous, incorporeal undead in the Dark Plane haunt the place, and there are patches of the forest that are nothing but Twilight. Anyone that accidently stumbles into a patch of Twilight is thrown across the planes.
Cave of Mysteries
The Cave of Mysteries is home to many of Saetan's twisted creations, his "children". In the Cave, spells below level 4 do not function, and those not bearing the Black Mark are unable to speak coherently (making them unable to talk or provide a somatic component to a spell). The Cave also houses one of Saetan's most powerful non-living creations - the Pool of Shadows. The pool was originally a natural warm spring, given potent enchantments by the Black Prince. The effective arcane caster of any evil creature who bathes in the pool is increased by +3. Should a good creature bathe in the pool, their caster level is reduced by -3. Immersion in the fountain's waters confers the benefits of a heal spell for every round of immersion.
The Everdark Dungeons is comprised of 66 prison cells. Each cell is under the effect of a permanent antimagic field an a permanent Assassin's Darkness spell (anyone bearing the Black Mark can see through the darkness). The walls, floors, and ceilings of the cells are solid, made of adamantine and encased with iron. On command from a jailor or the Black Prince himself, the metal chamber becomes red hot, and the inhabitents of the cell are subject to a healing spell every round - a grisly form of torchure. On command from a jailor or the black Prince himself, the inhabitents are subject to a hold monster spell DC 30 every three rounds.