My last post dealing with Psychics I made mention how they draw on the Astral Plane to power their mind abilities. In the Onn Core Rules I explain how magic draws on the Ethereal Plane to power magic. This brings me to Onn's Planar Cosmology.
In the center of all is the Mortal Realm (or Prime Material, for those more familiar with that terminology). It is the plane all others are anchored to and indeed, there may be infinite alternate dimensions or realities of Mortal Realms where differences may range from simple and unobtrusive to wildly different and unrecognizable.
Between the Mortal Realm and Elemental Realms is the Faerie Realm and Twilight Realm.
Next come the Elemental Planes of Air, Earth, Fire and Water.
Outlying even farther are the Planes of Entropy and Life.
Even farther out are the Realms of the Gods, the Realm of the Dead and other Planes (such as the Prison Realm).
The Ethereal Plane surrounds and connects all of these planes together, providing connections and weaves them all together. This is also the Plane of Magic, where spellcasters draw their powers from. Only through the use of magic can one enter the Ethereal Plane.
The Astral Plane also touches all other planes and is concurrent with the Ethereal Plane in existance. Spellcasters cannot use magic to enter this plane, only a highly psychic mind can project itself into the Astral. In addition, it connects the Dream Realm to all other planes. The Astral though reaches a point beyond the most distant God's Realm that ends in a barrier that most percieve as a massive Astral Whirlpool. Creatures entering it have never been seen again (even Deities). Occasionally exceptionally powerful creatures that should not exist in 'normal' reality come from beyond the barrier and using the Astral Plane, make their way to the Mortal Realms to wreak havoc.
What happens to one's spirit on death is a subject open to debate. The most popular theory currently among sages is divine spellcasters are taken to the realm of their Patron Deity, arcane spellcasters become part of the Ethereal Plane, Faerie creatures (such as Elves) become part of the Faerie Realm, Psychics become part of the Astral and the rest travel to the Realm of the Dead. Of course this topic, while hotly debated, would require the death of the theorists to prove who was right (and of course, they're not in a rush to prove anything!).