The Astral Sea
The Astral Sea, known also as Astral Plane, is the plane of thought, memory, and psychic energy as well as one of the two fundamental planes. Held tightly within it are the dominions of the gods, and at one point before the Disjunction even the Nine Hells, while the open spaces in between it is where gods go when they die or are forgotten and what was once mistaken for the Astral Plane in its entirety, a barren place with only rare bits of solid matter. The Astral Sea is unique in that it is infinitesimal instead of infinite; there is no space or time here, though both catch up with you when you leave. The souls of the newly dead from the Prime Material Plane would once pass through here on their way to the afterlife or Outer Planes. Ever since the Disjunction however and the planes of the Material world and many other become one, the dead still come here when they die but are capable of leaving thus no one truly dies any longer since the plane of the dead and the living have merged. (Thus if your pC dies it is not always the end and they can just pop up here and you can start using them again)
A common feature of the Astral Sea is the silver cords of travelers using an astral projection spell. These cords are the lifelines that keep travelers of the plane from becoming lost, stretching all the way back to the traveler's point of origin.
It should be noted while those that die can end up here, only the pure of heart and intentions stay, those who's hearts where judged wicked or where if evil alignment instead go to the abyssal or Nine Hells.
The Fugue Plane is a neutral plane within the Astral Sea where the souls of mortals are drawn when they die. The vast majority of this plane is flat, gray, bland and nondescript, with no notable topographical features.The sky overhead is also gray.The plane's only significant feature is the City of Judgment in the middle of which stands the Crystal Spire where Kelemvor and Jergal reside and it serves as the former's dominion. There are only three methods of getting to the Fugue Plane from outside the astral sea: as a divine servant collecting a deity's future petitioners, through non-permanent portals from the Nine Hells or the Abyss, or through death.
The Nether Realm
Netherrealm appears to be like The Astral Sea, a realm composed of thousands of different layers however this is where the similarities end abruptly. It is here that deceased souls or entities who have committed major crimes descend into the Netherrealm to undergo endless torture. In fact, one cannot enter the Netherrealm without being evil or having a soul tainted with evil. Additionally, magic users and magical beings are slowly weakened while in this realm. Those who draw on dark pacts with demons or devils are not effected but have a increase in some of their powers. Demons, devils, and fiends are also much more formidable here.
The Netherrealm is in fact the merging of both Limbo, the abyssal planes, as well as the Nine Hells. Much like the Astral sea those who perish in the new world of Terra Nova come here after death, and only have a small sliver of hope of ever escaping. Those who where evil are sent to the Nine Hell but those who where of neutral alignment instead end up in Limbo, which is centered between the Abyss and The hells.
One major point of interest is the Blood river, for at the end of this river is said to be a relic of immense power. The blood crystal. A artifact born of the endless centuries of conflict between the devils and demons, it is said to hold the soul of every one of those accursed fiends who died in their endless war and is one of Terra Nova's most powerful items. However, perhaps by some great providence, it is a power far beyond the reach of any mortal or being who would wish to use it for their own deeds.
The Nine Hells
The Nine Hells, also known as Baator in Infernal or simply Hell or Hells, is the home of the devils.
Baator is stylised as a plane of sinister evil and cruelty. The different types of devils that dwell here obey a strict hierarchical social structure. Each continually plots to advance their position through treachery and deception. Unlike the demons of the Abyss, the devils are highly organized, with a logical and calculating nature.
The plane itself is composed of nine different layers, each of which models a differing but no less inhospitable and dreadful environment, including barren plains of ash and rock to frozen wastes of endless ice. The nine layers of Baator are as follows:
The first layer is a vast charred wasteland of rubble over which the iron towers of the Dukes of Hell stand. Legions of devils march across the plains in continual readiness for the next battle of the Blood War. A red light suffuses the sky and huge fireballs fly across the layer, randomly exploding wherever they hit.
The second layer, Dis, is a burning city of iron known as the Iron City of Dis. The walls of the buildings of the city radiate extreme heat, as do the stones of the streets; more than brief skin contact results in severe burns. The Archduke Dispater rules this layer from the Iron Tower, an impregnable fortress that reaches far into the sky and can be seen everywhere on Dis.
The third layer of Minauros is an endless bog of vile pollution. The weather on Minauros consists of acidic rain and harsh winds.
The city of Minauros the Sinking is located here, so called because the weight of the city causes it to continually slip beneath the slimy waters; only the endless efforts of thousands of slaves prevents it from doing so. The kyton city of Jangling Hiter, City of Chains can also be found on this layer.
Mammon the Viscount is the ruler of Minauros, he resembles a long serpent with a human torso. He rules from the centre of the city, within a huge mausoleum like structure.
Phlegethos is a fiery wasteland filled with active volcanoes and rivers of molten lava. Lady Fierna and Archduke Belial are the lords of this layer. Few can exist here for long without suitable protection from the extreme heat. The city of Abriymoch is composed of hardened magma in the caldera of a volcano and is somewhat more bearable than the rest of Phlegethos. Fierna and Belial reside in a palace of pure obsidian high up on one side of the city.
Stygia, the fifth layer is a freezing layer of cold and ice dominated by a murky ocean fed directly by the River Styx. The dark sky is constantly filled with lightning storms. Tantlin, the City of Ice, is built on a huge ice floe with a large harbor to the river Styx, and is ruled over by a huge pit fiend. The lack of any kind of law enforcement leads itself to gangs controlling much of the city. Prince Levistus rules over this layer, frozen in a giant iceberg floating in the harbor.
Malbolge was an endless mountain where rockslides and avalanches of boulders were common. Numerous copper fortresses provided some refuge from the avalanches. Formerly, the Hag Countess ruled from her mountain-sized boulder fortress, which perpetually rolled down the slopes of Malbolge. Now however the layer is ruled by Glasya, the daughter of Asmodeus. The Countess grew to such a height and girth that she exploded and became the layer itself.
Maladomini is a barren landscape scarred by countless forgotten ruins, abandoned mines and gruesome subterranean dungeons. The city of Malagard is the layer's primary settlement. Archduke Baalzebul presides over this layer.
Cania is an unimaginably cold wasteland, few living things can survive more than a few moments in an unsheltered area. Huge fast moving glaciers crash around the layer sending snow and ice up in the sky.
The layer is ruled over by Mephistopheles, from his great citadel of ice Mephistar. It sits upon a giant glacier called Nargus, the movement of which is controlled by Mephistopheles himself. Inside Mephistar huge heated baths and fire warm the citadel, providing quite a comfortable environment.
Nessus is the ninth and deepest layer of Baator. It is of pits and ravines of virtually endless depths. The great city of Malsheem lies immediately below the layer between Cania and Nessus and is the largest city in all of the Outer Planes. Here, Asmodeus rules over the entire plane, and thus the entire race of devils.
The Abyss, sometimes more completely called the Infinite Layers of the Abyss features an endless variety of evil. Shaped as an endless number of layers, one atop the other, the Abyss stretches into infinity, though some Terra Nova scholars have theorized that the plane has no more than 666 layers. But this is unknown, as no adventurers have been known to return from anywhere below around the two hundredth layer.
The only constant in this chaotic realm is the presence of the adjacent Blood Rift where the demons fight their mortal enemies, the devils. The proximity of the Blood Rift to both the Abyss and the Nine Hells, along with Limbo, allows for the fiends, demons and devils to wage an unceasing war upon each other. But unlike the Nine Hells, which is always connected to the Blood Rift by its top most layer (called Avernus), the ever shifting nature of the Abyss brings different layers into proximity with the Blood Rift at different times, allowing the demons to constantly bring fresh forces to bear in their constant struggle known as the Blood War.
Limbo is a large realm of varying landscapes, though comprad to the other two merged planes The Hells and the abyss, it is quite small. Limbos primary landscapes appear to consist of, mostly barren wastelands, deserts, pools of acid, dense forests with animated faces, and high, mysterious mountain peaks. Small villages are scattered throughout the landscape, and like the Astral sea, the realm teems with magic and is the only place in this new plane now called the Netherrealm that magic user's are not drained of energy over time. Fiend's are the main beast of this plane and are more wide spread then any other race. The entire land is centered directly on the blood river much like an island of sorts, because of this it is the major center point of the blood war, with huge scale battles between devils and demons a daily occurrence. not that that one can easily tell if it is day or night in the Nether realm.
Limbo is the place that deceased dead go to who either held no faith of any kind or where nether 'evil' or 'good'. While it was once a place of beauty, it has become a wasteland partly due to the merging with two other chaotically evil planes and partly due to the war that is now waged across it's entire surface, even the once peaceful and beautiful Seraphim have become twisted hideous beast called 'Fiends'.
The Nether realm is the prime example of a plane merging gone horribly array...or horribly right.