Cosmology (Lands of the Twin Gods Supplement)
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 The Great Source
 The Demon Realm
 Elemental Planes
The Elemental Planes existed before the gods descended from the Great Source. The gods used part of the Elemental Planes to shape the world. The ahrneks that lived in those parts of the Planes resisted, making war against the hiisi that followed the Gods to their creation. The djinni and their servants, however, did not care about the actions of the gods aside from being mildly annoyed.
Since before the dawn of time, demons have tried to invade the Elemental Planes. The demons come from a realm no one has ever visited, a distant plane inimical to life (some believe that their home plane is the cosmic opposite of the Great Source). The creatures of the Elemental Planes have always had to fend off demons in between petty wars against each other. Since the creation of the mortal world, the demons have tried to attack it with equal gusto.
At the figurative "top" of the Elemental Planes is the Samsarga, thousands of demi-planes hanging in divine space, each are a purified element, be it a plane of mercury, gold, glass, sandstone or blue gaseous flame. The planes are the easiest to travel of the Elemental Planes, if only because two large planes, the Realm of Light and the Realm of Darkness, orbit the space and connect to each plane in turn, thus providing some form of time. Within these pure planes live the Djinni, regal and aloof lords of the elements. Having access to the many "Treasured Planes", they have infinite material wealth.
 City of Brass
At the center of Samsarga, or so the efreet of the djnni like to think, is the City of Brass. This is a demi-plane made entirely out of elemental brass that has been carved using magic into a near infinite city. Over the many thousands of years it has existed mountains of other materials have been imported, so much so that few know it was once a pure elemental plane like the others around it. It is considered the oldest city in existence. It's grand bazaars is truly grand, as it should be, in a city where the salesmen can grant wishes!
The heart of the Elemental Planes is the Himiga, know as the Elemental Chaos to most, a infinite expanse of rolling elements, here fire, earth, air and water are unbound and move over each other freely. Here simple minded, but colossal elementals reign supreme. Once in a while a area of the Plane will calm and stop moving, this can go on for centuries, but eventually a elemental will notice and cause it to move again. Storms blow, the earth crumbles, volcanoes erupts and Maelstroms roar.
Numbiga lies at the bottom of the Elemental Planes. Here the elements are totally interchangeable, thick boiling ooze and dark sand is all that can be seen between the thick ash and rancid fog. Here, much to their charring, live the Ahrneks. These creatures could of been the Hiisi of the material plane, but resentment of the Gods might and an innate desire to cause discord was there downfall. While they are much despised by the other beings of the Planes, they are the frontline against the Demons.
 Vul Gate
While technically not a part of the Elemental Planes, the Vul Gate is a crack into Numbiga which the Demons use to enter the Planes. Demon camps circle the Vul Gate, fortresses made of brittle clay and shards of rusted metal and whatever they can get their hands on. The Gate itself seems to lack a physically presence, as if it is "made of nothing".
 Parasite Planes
The Feywild, or Tiranog, the "World to the Right". It is the realm of verdant, vicious to the extreme, endless forests home to primordial beast; like the tyrant lizard and the sabre-tooth, the flawless Fair Folk in their fairy mounds that are all at once hearth and marble tower and the tyrannical fomorians deep within the coal-blacked ground. It is a plane of innate magic and untempered abundance. It is the home of trickster Gods, of Hiisi, dryads and nymphs, inhuman eladrin and any number of creatures that feed on dreams and superstition and flesh. Lots of flesh.
The Tiranog is a plane of wild perfection and crazed wishes, so much so than few that can survive here choose to leave it, they become part of the plane, often twisted by the plane to suit it and just forget their home. Being stolen away is a dreaded thing by many elves...though, well, some choose to loose themselves. That’s another matter. Mirrors and crossroads are known to lead to Tiranog if one stands near them in the right sort of twilight.
In contrast, is buzzing, bastard Sigil, the "World to the Left". Here a equally endless, and just as vicious city covers the torus, where beasts are really not the problem, the philosophers are. They have clubs. If the creatures of Tiranog feed on fables and grow on mystery, Sigilians live by wits, street smarts and a good, long conversation about life, the universe and everything that meanders into a argument and then into school-boy name calling (of which they above all, are the best at).
The world is controlled (or not controlled) by hundreds of Factions, each with its own views on the planes, they also have duties outside of these debates and research missions that keep Sigil ticking, such as the Guvners, that often acts as judges, lawyers, stockbrokers, mathematicians and anything else that is sinisterly law abiding yet law bending.
Again, in contrast to Tiranog, no Gods or high spirits of any kind can enter Sigil and indeed, it does not trap visitors, but opens doors to the infinite multiverse, as a portal can be opened in Sigil to anywhere, if you only have the right key.
 The World Below
The World Below, the Shadowfell...is as it is already stated in core. It leads to the Afterlife through many means, the most known being the Glass Desert. It is bleak and dangerous. Wells and basements are said to be paths into the World Below.
 The World Above
[Oh and in a general sense, important things come in twos in this setting. I think. So the realm of the Gods must be The World Above. I think it'll be quite cool to have the actual house of the Gods being able to meld with the material plane, even if only rarely.]
 The Worlds Inside
"If there are The Worlds Outside from where demons, elder gods, and anything else unimaginable by mortals -and thus unshaped by mortals' dreams- came from, then what is The Worlds Inside?
"If one look at the deepest of their mind, deeper than Where The Soul Lie, one will find pathway to The Worlds Inside. Into this place the mortals' dreams, imaginations, memories, and whatever else come sipping through the gaps in the Wall of Separation and forming into something indescribable. It is everything you can imagine, and yet it is so vast, deep, and compels that no single mind can comprehend the nature of its being. Everything that made it are come from the minds, which make it unreal, but the place itself -if you would believe me- is truly real.
"Here lies the Gate to What Has Existed Before Everything Else. . Rarely does it opens. When it does open, a soft, yet powerful Wave will come from it. Like how earthquakes change the shape of the land, The Wave changes the shape of reality. It will propagate to even beyond the Worlds, carrying part of The Worlds Inside in its wake. At the end of The Wave, what previously is only an amalgam of imagination come into being. This is how Feywild, Sigil, and other Echoing Worlds created.
"Some pretty good nonsense, don't you think? I heard it from that insane bard in the town square".
The fleeting World Inside exists at the crossroads of people's inner minds. The Amuntest called it The Dream Time. Creatures here are short lived (often as short as a person's dream) and driven by simple, if abstract goals.
The few dream-creatures that live long lives are called Oneiros, which are said to be dreams that have taken on a life of their own and the Somnians, dreamers who, instead of dieing, fell into eternal sleep and now live only in the dreams.
Several Gods travel though this realm, though only a few make their abode here.
Hiisi (and many other beings) can not enter this realm at all, as it has no connection to Nature.
The World Inside has a nearly limitless concentration of energy churning within it, created by the minds of those who visit it during dreams. Dream Walkers, a sect of Mages, have learned to implement magic to reach the World Inside while in a state of trance. While there, they can harness said power and use it as a magnifier for their divination magics. Some are even able to take control of the bodies of others that are currently connected to the World Inside, and use them to their own devices.
 The World Behind
Also known as Mechanus. The great engine that drives the wheels of the physical world. It is a world of cranking gears the size of mountains, pistons larger than trees, and forges of flame that never goes out. It also serves as the home of the Inevitables. This is a race of metal men similar in appearance to the Warforged, yet vastly different in purpose. The Inevitables were created as custodians to the natural order of the wheels of existence. If someone were to throw off the mechanics of the universe (such as cheating a Death Hiisi one too many times, or using magic without undergoing equivalent exchange) they are required to bring equity through eliminating the offender. Inevitables play music, vast operas and orchestras, all of which are made using intense mathematical equations. They are always in perfect time. It is beautiful music and can go on for months, then they abruptly stop and continue going about their business.
Mechanus is connected to Byd by giant screws in the iron band at the center of Byd's torus. It is only reachable through journeying down to the core of the world and finding one of these screws.
In ancient times an angel known in legend as Anon and the devil Bathe were said to have fallen in love. Anon was a servant of Ashore and Bathe was the daughter of a high ranking Riding Duke, they fled both of their homes and finding no save place for themselves, they bartered their services to the demon Tarathur. Perhaps in modern times this would of been ignored by the Prince, but having an angel and a devil in his legion was something to grin about and ask the other Princes if they had such a pair. They did not of course. In return for fighting in the grime and smog by the Vul Gate they were given rule over a section of the demon's impossible plane.
But devil's are cunning and angel's are creative, in the absolute sense. How they did it is unknown, but they broke off their land, which they dubbed Pandemonium, and to this day it is said to float between worlds, ever evading whoever would stop their love. Some legends speak of them offering rebels of all kinds a place in their land that ignores all rules. Some speak kindly of Pandemonium, others not, perhaps it all depends on if you meet Anon or Bathe upon your entrance Some tales speak of an alliance between the couple and the Oni, some even say it was they that turned them away from their Prince and that Oni can enter Pandemonium at will...
 Jubilex, the World Apart
The World Apart was once a bleak and dismal parasite plane where the souls of those who died utterly alone, stillborn or by a great betrayal by a loved one crawled blindly in search of companionship. These souls so longed to feel the presence of others that they became trapped between Byrd and the afterlife. Shereb'Shem saw these souls and in her infinite love reached out, blending the souls together into the great amorphous and blissful plane of Jubilex. All oozes are a part of this plane and are created when a portion of this plane extends into Byrd, primarily at the site of a death of great loneliness or after the death of one who was killed by an ooze.