Abmarah is but one of the many planes in the greater cosmology of 'Voidverse'. Similar to the cosmology of other Dungeons & Dragons settings where different realms are suspended in an Astral Sea, Abmarah's cosmology follows a world-tree pattern of nine realms and many planes within those realms. All the realms, planes, and the astral sea combined are together referred to as the Cosmos or the Universe. All living beings in the Cosmos are inherently divided into three realities: Order, Chaos, and Balance between them. Some, however, are mysteriously neither—a Void. This theory of splitting reality into four categories is part of the creation myth of the cosmos.
The Nine Realms
The Prime Realm
Containing the material plane as well as the combined planes of the Feywild and Shadowfell, this is where most of our stories take place. It is surrounded by the Ethereal Realm as well as Voidspace, the solar system Abmarah is a part of, leading into eventually becoming the Astral Sea.
The Feywild acts much like the Feywild as described in the standard cosmology of the Dungeon Master's Guide or other sourcebooks, with the exception that the plane is connected to the Shadowfell which is very different compared to the traditional D&D cosmology. The Shadowfell and Feywild here are two sides of the same coin, and you can essentially walk from one to the other.
The Realm Beyond
Also known as the Afterlife, the Realm Beyond contains all the afterlife planes. Souls of the dead wander through the desert of the Fugue Plane that surrounds the realm, and then pass through the veil to the neutral plane of Azydos. Judged by Theratis, they are then sent to one of the Nine Heavens or the Nine Hells.
From 'top' to 'bottom', the planes are Araboth, which contains Jakuna's Court, followed by Jovarde, Elysium, Valriki, Svarga, Tuwarea, Qaydar, Saturia, Asphodel, the neutral City Azydos, followed by the hells; Acheren, Vaitarnus, Duzakh, Phlegethos, Styxia, Gehenna, Meigoku, Letheos, and finally Tartarus, the realm of Krros.
The Rivers Beyond
Styx/Acheron: The River of Dread Oaths and Woes, any oaths made upon its waters are bound by divine law. Possibly the most famous river of all, it is also the longest. It flows from Jakuna's Court in Araboth, through green Elysium and the meadows of Saturia, through Asphodel, passing through Azydos and the Palace of Theratis, through Acheren, cooling down in icy Styxia named after it, and finally reaching the bottom of the realms in Tartarus having mixed with all other rivers in the process.
Phlegethon/Duat: The River of Fire, it is a flowing stream of fire and magma. It is said to flow from hot Jovarde, passing through Azydos, through Acheren, and finally reaching the lava-hell of Phlegethos.
Lethe/Gjöll: The River of Forgetfulness, any who drink from it lose their memories based on the amount they ingest. It is used to erase mortal souls of their entire memories so that they may be reincarnated or live without woes. It is said to flow from cold Valriki, through the ocean of Tuwarea and the caverns of Qaydar, passing through Azydos, and finally reaching the ocean-hell of Letheos where its waters are used as punishment instead by fiends.
Vaitarani/Hubur: The River of Purification and Sins, the rightful see its water as nectar while the sinful see it as a river of blood. It is believed that drinking from it can purify sins but falling into it can destroy sinners apart. It is said to flow from golden Svarga where souls drink from its nectar, through the caverns of Qaydar, passing through Azydos, and finally reaching the depths of Vaitarnus where sinners and fiends are punished forever.
Sanzu/Cocytus: The River of Poison and Wailing, this green river is infested with snakes and to drink from it is poison. It is said to flow from lush Saturia, passing through Azydos, through stinking Duzakh, and finally reaching polluted Meigoku.
The Elemental Realm
Similarly, the Elemental planes and chaos remain largely unchanged from other settings. With passages between the Plane of Air, Water, Fire, and Earth, they are all connected to each other through spaces like the Frostfell or the Plane of Ash.
While small gateways are created and destroyed randomly across other planes, there are four large primordial gateways between the elemental planes and Abmarah. The Primordial Gate of Fire is in the Km'mala Peaks on the continent of Eqos, the Primordial Gate of Water is underwater in the Aethian Dominion, the Primordial Gate of Earth is in the Tianling Range on the continent of Eteru, and finally the Primordial Gate of Air is in the Kosmosi Mountains on the continent of Aurial.
The Subconscious Realm
Emotions hold power, and the minds of mortals connect to this realm of subconscious planes. Based on Jungian philosophy, The Collective Plane contains the collective subconscious of all mortals, shaped by public perceptions and culture. Similarly, the Dreamplanes are the lovecraftian places mortals visit when they dream. Home to good and evil dream spirits and pockets of reality where dreams play out.
The larger part of the Subconscious Realm is the famed Plane of Madness, where aberrations originate from. Similar to the Far Realm of the standard cosmology, it leaks out and connects to the Astral Sea as a whole.
The Realm of Chaos
The physical manifestation of the reality of chaos, it has shrunken in relative size due to the orderly nature of the current era. It holds two planes, the first of which is Shatterrealm, a hostile realm of varying landscapes. A composite realm of shattered lands from other realms, it is a chaotic mix of merged and conquered realms. Sands mix with snow while pools of acid can mix with lush alien forests.
The second chaotic realm is Limbo, the ever-changing chaos as described in the standard cosmology. Composed of supreme chaos, sentient beings such as Githzerai can carve out small pockets of safe zones to call home.
The Realm of Order
The physical manifestation of the reality of order, it has expanded in relative size due to the orderly nature of the current era. It holds three planes, the first of which is the Daylands, a realm of calm forests and contained lifeforms. A perfectly planned endless forest, it is life distilled down to its least chaotic nature. The second plane is Mechanus, the orderly realm of Modrons as described in the standard cosmology.
The third is the Dimensional Planes, a series of infinite dimensions beyond the three dimensions that we can fathom, where true indescribable order reigns. Aethys' Isle of Souls exists outside the Realm of Order, a small garden island that floats above the Daylands as a demiplane where the golden goddess resides.
The Realm of Balance
The physical manifestation of the reality of balance between order and chaos, it has been defaced and left imbalanced in this era of relative order. It holds two planes, the first of which is the Ruin of Celestia. Celestia used to be the home of the gods in the first era and prior, similar to the Celestia described in the standard cosmology. However, the Omniborn Crisis in the first era saw its destruction, as the gods left the Realm of Balance in ruin and took home in the nine heavens of the Realm Beyond.
The second plane is the Archive, which was formerly a verdant endless forest in the primordial era. Corrupted by Gunian the Archivist, it has become an endless eldritch library, where almost all the knowledge of the divines and mortals alike is stored by the Keepers. Travel to the library is relatively easy, and Gunian offers no charge to access most of the knowledge stored within. To take from the archive, you must present Gunian with new knowledge he does not yet know of—or a look into your memories.
The Voidal Realm
The physical manifestation of the lack of order and chaos and of the indescribable void that remains, the Voidal Realm is the farthest realm in this cosmology and the one full of indecipherable complexity that mortal minds aren't meant to comprehend. It contains the Negative and Positive planes as described in the standard cosmology, as well as nine monolithic planes.
The Monoliths are an ancient eldritch concept, akin to gods but much older. Some say that the Monoliths are outside of time, and are from the cosmology before this one. They have nine planes of different purposes here, such as Xor, Arkthys, The Nightlands, The Chamber of Visions, The Maze of Stars, Tindalos, The Foglands, The Scrapheaps, and Faro. These planes are not meant to be visited, or even known about.
The Abyss
The ninth and final realm of this cosmology is the infinite layers of the Abyss, as described in the standard cosmology. Here, it was said to be created by the god Varkanus, and the malevolent home of demons constantly shifts and wars with other planes.
The neutral City Azydos of the Realm Beyond has a large pit that connects to the Abyss, where the gods throw the worst of mortalkind that do not deserve even the ninth hell.