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A Complete Guide to the Cosmos - A further look at the Multiverseby Giulio Caroletti
As mentioned earlier in this series of articles, there are several Prime Material Planes, all sharing the same Ethereal and Elemental Planes, although each Prime Plane has its own 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.
There is only a way to pass from one Multiverse to another: through the Prime Planes, or, more exactly, entering the Ethereal from a Prime Plane. Thence, one has to travel far until it reaches the Deep Ethereal. Deep Ethereals are the four-dimensional areas where the boundaries between the Prime Planes are not present, so one can pass through it and reach another Prime Plane. The problem is that only True Gods or immensely powerful Immortals or Athanatoi are able to know exactly where to go to reach the desired Plane; mortals and immortals alike risk to lose their way in the trip through the Deep Ethereal, or to die due to the dangers present there.
Only the True Gods, however, know that there are several Prime Planes, all other creatures believe that there is only one Prime Plane and that the others are Demiplanes. This feeling is enhanced by the presence of gates, in the Ethereal, that sometimes allow any creature to pass from one Prime to the other. Most of these gates are temporary. Lastly, there ARE also Pocket Dimensions (or Demiplanes), exactly like Prime Planes but without any Outer Plane. The only way to enter and exit a Pocket Dimension (or Demiplane) is through the Ethereal's Gates - there is no way to reach them from the Deep Ethereal. Most of these Pocket Planes/Demiplanes are known by powerful planars and godlike beings, further enhancing the confusion and preventing all of them from knowing the real situation of the Cosmos.