Cosmology of Oerth Volume One- A Short Discussion on the Nature of the Planes

Nature of the Multiverse
Beyond the realms of the physical world lie a myriad of alternate realms of existence. Often called the Outer Planes, these realms are of various sizes, shapes and natures. Each can have wildly divergent physical laws and are shielded from other planes by the spiritual ether. The outer planes shift around each other in random paths, making it difficult to accurately map the multiverse.
While the multiverse is nigh impossible to map traveling the Outer Planes is considerably easier. Besides the very reliable spells and portals found throughout the world one can also travel the trods. Trods crisscross the multiverse, tracking along the borders of many planes. One must simply step off the trod to enter the plane they desire. Unfortunately, since the planes are constantly shifting the destination of the trods change on a regular basis. It is impossible to know if a familiar trod will take you where you desire or to some alien realm where the laws of physics have gone horribly wrong.
But since the trods themselves never change location on the physical plane travelers may consistently enter them in the same place. Trods are often found in places of great power like stone circles or nodes of mystical energy. They are also found in the lonely places of the world; desolate game trails, lonely heaths, even in the bottom of a lake. Rarely are trod entrances found in urban centers. Finding a trod often requires the use of spells and just because you find a trod does not mean you can immediately enter it. Rituals are required to gain entrance to a trod and this ritual is unique for each trod. For example, there is a trod found in the Lortmil Mountains that requires the sacrifice of an animal to open it. How a trod entrance operates on an Outer Plane largely depends on the nature of that plane.
It has been postulated that the number of different planes of existence are infinite. While traveling one can find planes of pure elemental power, infernal planes, heavenly planes, planes of chaos, planes of law, and planes that mimic the appearance of Oerth. In fact, all of the alternate Oerths are generally believed to be another part of this complex planar structure. Since there are so many different planes broad categories have been established to help define the nature of the multiverse.
When looking at the planes of existence it must be determined if the plane is:
1) An alternate material plane- Often these are planes exactly like or very similar to Oerth. In fact the world called Aber-Toril could be considered an “alternate” of Oerth.
2) An elemental plane- Planes that have an overriding affinity with one of the traditionally recognized elemental energies. Earth, Air, Fire, and Water are most common but planes of Wood, Acid, Ash, and Sonic Energy have also been discovered.
3) A demi-plane- Demi-planes are small planes, usually with clearly defined boundaries. Often created by gods or powerful mages for very specific reasons, demi-planes come and go like leaves in the wind. Some can be bound to a particular location, in effect sharing the same space as another plane.
4) An infernal plane- Also called Hell Planes. These planes are the homes of demon, devils, and other infernal creatures. They can be horrifyingly chaotic or disturbingly orderly.
5) A celestial plane- Also called Heavenly Planes. These planes are the homes of celestials and angels. As with Hell Planes they can be chaotic or incredibly orderly.
6) An ideological plane- Ideological planes have an overriding affinity for a particular ideal. Lawful, chaotic, good, and evil are just a number of the options available.
7) A paradigm plane- If you have dreamt about something important to you chances are it found its way to the Outer Planes. Thoughts have power and the multiverse will often create situations for those thoughts to exist. Paradigm planes are more fleeting than demi-planes because they require the power of belief to remain real.
As with all things, strictly categorizing them will cause problems. Do not be surprised if you run into something that does not fit these classifications.

Ascendancy of the Planes
Due to their very nature it is possible for planes to occupy the same physical space. Planar sages call this ascendancy and spend most of their lives attempting to predict these events. Very few rules govern ascendancies and it is often impossible to predict how long one will last. What is known is that when two planes are in ascendancy they influence each other in subtle ways. More than two planes can be in ascendancy at the same time.
The most recent, dramatic ascendancy can be seen in the Greyhawk Wars. Right before the wars three infernal planes came into ascendancy. These planes did not move out of their ascendancy until the Flight of Fiends. While they were here the Flanaess experienced unprecedented bloodshed. It has been postulated that the Rain of Colorless Fire and Invoked Devastation happened during a similar period.
Currently Oerth is enjoying the beginnings of a celestial ascendancy mixed with an elemental plane of Fire influence.


Volume Two- Specific Planes known to Oerth.