The Metaverse is a multiverse cosmology designed to allow for crossovers between various fantasy campaign settings, as well as allow for a variety of methods of traversing the planes including fantasy space adventures, portals and planeshifting.
The Metaverse has three fundamental planes: the Plane of Shadow, the Astral Plane, and Tarassein. These can be imagined to exist as concentric cylinders with the Plane of Shadow on the inside, the Astral Plane in the Middle and Tarassein on the outside.
The Plane of Shadow is a lot like our real world space. In its deepest regions, it bears some resemblance to our world’s space. This Deep Shadow, as it is called, can be traversed using a specially equipped skysailing vessel, and used to reach completely different cosmologies with their own planes, gods and mortal worlds. The region where the Plane of Shadow borders these alternate cosmologies is known as the Border Shadow, a region which starts to take on the traits of the plane it border.
Surrounding the Plane of Shadow is the Astral Plane, an impossible infinite plane. If the Plane of Shadow connects the “matter” of the multiverse, then the Astral Plane connects the “ideals” of the multiverse. Many outer planes and divine realms are accessible from the Astral plane, which can be traversed with powerful magic, portals, or (again) with special skysailing vessels.
Encompassing the whole of what has been mentioned before is Tarassein, a realm barely understood, and indeed impossible to understand for rational minds. It is a realm of chaos, and it is the source of many aberrations in the multiverse, including the tarotic aboleths. Some sages speculate that is like a disease on the fabric of reality, others worry that it is actually the worlds of humanoids and outsiders that are the disease, and Tarassein is just an immune response, a buffer, between a larger, more fundamenal, and more insane reality beyond.
While one of the major ideas of the Metaverse is that it is a multiverse connecting a myriad of fantasy worlds, it is important that these connections are handled in such a way that these worlds do not lose what is distinctive about them. Here are some rules of thumb to stop that from happening:
See Planar Gates
There are a few options for travelling the planes. Expensive rituals can be used to plane shift. Otherwise, the most common form of planar travel is skysailing.
Aboleths, Original Spell Name Compendium, Demons and Lawful Entities, Gods