Cosmology (Age of Titans Supplement)
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The primary difference between this cosmology and others is that the planes are not actually planes of existence, but are merely places that are extremely far away from each other. If you wish to consider this setting a part of Endhaven (DnD Campaign Setting) then it is consider a very large Planar Cluster.
The Material Plane
The material plane is a flat area in the center, thousands of miles of ocean surround the continent of Geburah and surrounding lands. The ocean turns to ice at the edges, but there are occasional waterfalls over the edge of the plane. The material plane is supported by a vast column which descends through the negative energy plane and into Hell; at the top it widens greatly as it connects with the underside of the material plane. The sun orbits the material plane, ascending to the vague bordering area between the material and positive energy planes during the day and descending through the negative energy plane at night. The moon follows a similar orbit, though it is somewhat closer to the surface of the material plane than the sun, and its orbit somewhat slower.
The Elemental Planes
After thousands of miles of empty expanse, to the north, west, east, and south of the material plane are the planes of water, fire, air, and earth, respectively. These are not visible from the material plane because they are several hundred miles below its horizon. Where the energy planes meet they form chaotic mixtures of their elements: steam (water and fire), smoke (fire and air), dust (air and earth), and mud (earth and water). These intermediate energy planes are considerably smaller than the pure energy planes, but still a larger area than one raised on the material plane can adequately conceptualize.
The Energy Planes
Positive Energy Plane: Far above the material plane is the positive energy plane. This is a place of natural tranquility and healing. It is a realm of the stars, which are stable glowing beings of immense size and benevolence, only overshadowed by the light of the sun. Many flying creatures inhabit this area, all of which are immortal (unless they are killed). While in the positive energy plane there is no need for food or rest.
Heaven: Heaven is far above the positive energy plane. No one knows exactly what Heaven is like, though prophets have occasionally claimed to know. No one has ever visited Heaven and returned. Heaven is distinct from Celestia, and so is not the realm where angels and other good outsiders dwell. It is much more esoteric than that.
Negative Energy Plane: Below the material plane is the negative energy plane. This is a realm of chaos and pain. In the center is the pillar upon which the material plane sits. The pillar itself is the size of the entire continent of Geburah. It has a stairway snaking around its exterior and cavernous kingdoms carved within. Some plants glow on the ceiling (the underside of the material plane) which are eaten by small insects that form the basis of the almost entirely carnivorous ecosystem. The beings that live here are vicious and hardy, most with natural necromantic powers, as necromancy is easily learned and carried out here.
Hell: Far below the negative energy plane is Hell. Like Heaven, Hell is mysterious and esoteric; it is distinct from Baator. Also like Heaven, no one has visited Hell and returned.
The Outer Planes
Further still to the north, west, east, and south of the elemental planes are the outer planes, which is one giant disc which goes outward infinitely, according to most scholars. Also according to scholars, because it goes on infinitely, any possible place does in fact exist somewhere, and when a person creates a "demiplane" they're really just linking up to some place far away on the outer planes. Some have even conjectured that the material plane is simply part of the infinite outer planes and is not even really in the center. The outer planes are the realms of the gods, as well as many other mighty beings. Many regions on the outer planes have their own names, such as Limbo, Celestia, The Abyss, and others, but they are all connected in a geography of a scale only the gods could comprehend.
The Transitive Planes
The astral plane, ethereal plane, and plane of shadow are more closely resembling the classic planes of existence, but they are better interpreted as different ways of existing on the material or outer planes. This is explained by the philosophical concept of ontological relativity. The same world can be experienced and interacted with in a myriad of different ways, but it is always the same world. This also means that there are at least theoretically an infinite number of other transitive planes, but so far scholars of magic have discovered access only to the classic three listed above.
A few other planes have been discovered, such as the plane of dreams and the plane of mirrors. How they fit in is somewhat mysterious, but most dismiss them as just other places far away on the outer planes, despite some evidence to the contrary. For example, access to the plane of mirrors requires a mundane mirror, but what sort of place the traveler finds himself in depends greatly on who crafted the mirror, what it is made out of, and even how it is shaped. If the plane of mirrors is another standard demiplane, then it is unlike any other, at least in this respect.