The Planes of the Multiverse

There are many upon many different planes of existence within the Multiverse, of both ethereal and non-ethereal, ranging from Terr'ahna to the Abyss, the Divine, the Undergrave, Sacred Nexus, and so forth. These planes are all kept in balance and separated by a mysterious barrier, which is also a plane, called the Flow.
The Flow is like the bloodstream of the entire Multiverse, as within it flows all raw Mysia that exists. It extends between all other planes, and is as essential to the survival of the Multiverse as a bloodstream is within our own bodies. It is said that the Flow is the very source of all Mystia in our world, flowing into a network of crystal which is spread all across Terr'ahna. As in our world, raw Mystia is mostly found within crystals, and crystals are very much like batteries, able to hold a charge of energy for very long periods of time.
The Flow is essential to the Multiverse. It is absolutely necessary in order for the Multiverse to continue to survive, as without it, the other planes of existence would slowly merge and destroy each other. But because of this, it is constantly under attack by dark forces which seek only to destroy the Multiverse.