Cosmology (Dominaria Supplement)
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 The Transient Realms
 Sigil, The City of Doors
All of the planes are linked together in several strange and different ways. The first and most widely known is the mythical Sigil, the City of Doors. It is a city built into the inside a hollow doughnut-shaped structure of unknown origin and age floating over a vast, dark sea. Because there are no ways in or out of the structure, the city would be yet another planar anomaly but for the fact that the city is filled with portals to other planes, both within the multiverse and to the small worldlets beyond the Outer Planes. These portals are everywhere and easy to find…if you know who to ask.
Although Sigil is in and of itself the safest of all of the Transient Realms, it is a singularly contentious place: rife with factions and powerful entities struggling over control of the immensely valuable portals. Sigil is a natural gathering place for the many and varied ideological factions of the Multiverse, many of whose views would not be tolerated among the more civilized realms.
 The Rivers Styx and Oceanus
The Rivers Styx and Oceanus, one springing from Baator and the other from Elysium, travel between planes and serve to link the Multiverse together.
 The River Styx
Styx is a literal river that not only runs across planes but somehow between them. Styx connects every lower plane and the two middle planes, from Pandemonium to Mechanus. It is the largest means of transport within the deep reaches of the lower planes. Transport on the River Styx is crewed by beings called boatmen. The boatmen are thought to be a race that inhabited the lower planes before even the devils and the demons. They are the only ones who know the secrets of navigating the River. Aside from interplanar travel the River has a few other interesting qualities. Its waters are filled with the souls of recently deceased and induce memory loss. No one understands why or how this phenomenon occurs.
 The Boatmen
 The River Oceanus
Oceanus is also an actual river. Its headwaters start in the bottom layer of Elysium and connect all of the upper and both middle planes. Oceanus is one of the most widely used means of transportation in the upper planes. Transport along the water is usually handled by angels and natives of the upper planes. Boats traveling on the river need not be magical unless one plans on traveling between planes. Travel from layer to layer of one plane does not normally require anything extraordinary when on the river. The waters of the river are said to be endlessly deep, and are sweet, life-giving, and heal those who travel along it. Its banks teem with life, both plant and animal and these environments are often some of the most vibrant in the Multiverse. Souls from the Prime will often make their homes along the river.
 The Outlands
The Outlands are a large “island” floating in the Astral Plane. It looks like nothing more than a large piece of a planet's crust. It is a strange land with one portal to each of the Outer Planes – looking top-down, the portals are arranged in a manner similar to the standard diagram of the Outer Planes. These portals in the Outlands are well known in each of their respective planes and are heavily built up on both sides. In the upper planes they serve as large trade and commerce hubs as well as staging areas; while in the lower planes they’re garrisons and, on the planar side, are the sites of some of the most gruesome battles of the Bloodwar. Surprisingly, given the number of different factions represented, there is relatively little fighting in the Outlands themselves: most travel in the Outlands goes through any one of several outposts or forts built in the central region in the widely agreed upon neutral zone. The neutral zone is enforced by a strange race of constructs known as the Guardians. They are never seen unless someone physically tries to move an army into the neutral zone or a major fight breaks out. The only serious fighting that ever occurs is between the devils and the demons.
The general landscape in the central Outlands is mostly sandy soil with scrub grass. Scattered groves of African trees dot the landscape. As one nears the gates themselves, the landscape begins to shift to that of the plane that each respective gate leads to. There are many mountain ranges and lakes and even some more unique features. The Astral Plane that surrounds the Outlands appears to be a light blue sky full of wispy clouds. During the 'night'an aurora with the silvery-white trails of dimensional gates appears.
 The Endless Stair
The Endless Stair is yet another planar anomaly. It is, as the name implies, an endless stairway which leads from plane to plane in the Outer Planes. If one is looking at a traditional diagram of the planes going “up” on the Stairway takes one counterclockwise and going “down” takes one clockwise. Every so often on the Stairway will be a balcony or landing over looking the vast infinite space of the Astral Sea. On balconies or landings will sometimes be a door. These doors open as portals to a plane. Next to each door will be a painting depicting a vast landscape, somewhere within which the doors exit is. There will be several doors per plane on the Stairway. A short hallway will delineate the beginning of each new set of landings, which is wider than the stairway. Before the hallway starts there will be a plaque written in a completely indecipherable script. The hallway will have several large paintings on each side: these are paintings of various locales within the plane that this section of the Stair leads to. Each painting will have two small plaques on the bottom and right sides. Tubes on the ceiling filled with phosphorescent liquids light the hallways along with torches on the stair and chandeliers in the landings. There are no inhabitants, but one traveling on the stair can and sometimes does encounter other beings.
 The Plane of Mirrors
With the proper spell or magic item (for example, the mirror walking spell), a traveler can pass through the glass of a mirror or other reflective surface and enter the strange realm beyond. This realm is a semi-mythical demiplane that lies beyond the Outer Planes. It is a plane of endless hallways and corridors in a variety of styles ranging from medieval to Greek to even the late Victorian all of which is filled with mirrors. When one looks into a mirror for long enough, one eventually begins to discern the far side of a mirror somewhere in the Multiverse. It will eventually become clear that the mirrors are portals to another place. No matter what variety of hallway you’re in, it is always meticulously clean. The Plane of Mirrors can provide anyone who is willing to look long enough with an entrance to any plane. The entrance onto the other plane is always out of a mirror. The strong-willed character who knows exactly what mirror he wants to come through can choose where on the target plane he wants to come out. The only inhabitants of the plane are the mysterious Guardians: a hyperadvanced race of, as far as anyone can tell, sentient mechanical beings that completely ignore everyone on the plane.
 The World Ash
The World Ash is a gigantic tree sitting on a demiplane far outside the Outer Planes. The trees branches are wide enough to wage full-scale warfare, the bows shelter entire cities built right out of the wood, the trunk houses a honeycomb of passages leading up and down and across to other branches. This makes the wroshyr trees of the Star Wars universe look like weeds. Because of the immense distance between major branches, giant leaves are often employed as aerial sailing vessels. Smaller branches fork out many, many times from the larger ones and some lead to portals. Within the World Ash’s colossal canopy, branches lead to portals which lead every Inner and Outer plane. Portals on the Prime Material are often found by unsuspecting travelers deep within ancient forests or high up on an undisturbed mountaintop. This is the least known of the gateways between worlds.
 The Inner Planes
 The Outer Planes
 The Upper Planes
 The Lower Planes
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