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A Complete Guide to the Cosmos - the Multiverse and the Outer Planesby Giulio Caroletti
To fully understand the Nature of the Multiverse, it is necessary to make a sort of metaphor.
Take a giant plumcake filled with rosins.
The plumcake is the Ethereal. The rosins are the Prime Material Planes.
Now take this giant plumcake and put it into a container, divided into four zones, each filled with one perfumed essence. Through the plumcake, the essences in this container can reach all of the rosins. The four essences are the Elemental Planes.
In this container, apart from the essences, there are also two balls, one producing a positive electric charge, one a negative one (yes, I know electricity doesn't work this way, but - hey, it's magic). These two balls represent the Positive and Negative Energy Plane.
This is the Structure of the Inner Planes.
Until now, we have observed a three-dimensional space. Now, let's take a closer look at each rosin, and let's see what happens to the rosins if we see how they interact with something that is located in a fourth dimension, in a fourth space-direction (thus, without touching and interacting at all with what lies in the container or in the plumcake). In this fourth dimension, each rosin is connected to a point (and NOT A ROSIN) in another plumcake, through a "corridor" that is known as the Astral Plane. This plumcake is the whole Ensemble of the Outer Planes (simply called Ensemble from now on).
This time, however, when you reach the new plumcake from the old one, you'll notice that this plumcake is surrounded by a metal box that cannot be crossed. This metal box represents the Vortex.
It is to be noted that the Ensemble too is a single one, a single plumcake. However, this plumcake is divided into many smaller zones separated by the metal boxes (the Vortex), and thus it is completely impossible to reach one Outer Plane if you are not located in the Prime Plane that is connected to the part of the Ensemble that contains that point. This means that the only way to reach every Outer Plane is to pass through all the Prime Material Planes, and from here to a point of the Ensemble (a point in an Outer Plane) in the same area of the Plane you want to reach.
It is very important, at this point, to notice that while all "rosins" are connected to the Ethereal, not all are connected to the Outer Planes. Those that are connected to the Ensemble are the true Prime Material Planes. Those that are not represent the true Demi-Planes, Pocket-Dimensions, and so on. This is the main reason why most gods, planars and immortals believe in the existence of a single Prime Plane, and consider all others demiplanes. There is only ONE Prime Plane that leads to their group of Outer Planes!
A Prime Material Plane and its Outer Planes (plus the common planes of Ethereal and Elemental) is known as one Multiverse. Thus, there are several Multiverses, all sharing the Ethereal, Elemental and Energy Planes.
An example: one Multiverse is represented by the old D&D Game Multiverse (Spheres of Power and Immortal Home Planes, plus the Mystara Prime Plane and the common planes); another by the AD&D2E Game Multiverse (the Alignment Outer Planes, plus Sigil, and the Faerun/Spelljammer Prime Plane); another by the Myth Dimension (unknown Outer Planes, and the Mythological Earth's Prime Plane), and so on (Conan's world, Elric's world, Middle-earth's world, and so on and on and on).