I've thought about all this some more, and here is what I came up with:
- "Above" the Material Plane are the "Higher Realms" - the Astral Plane and the various planes and demiplanes floating in it.
- The planes and demiplanes are all sapient to some degree, and most are composed of a multitude of individual intelligences. A plane that has a strong "dominant personality" is often called a "deity", but there are many planes where no single personality predominates. Stil, even in these cases the compound intelligences must have something in common - like a common philosophy - or else the plane would fracture over time.
- Planes are able to grant mortals divine spellcasting powers, as long as the convictions of the mortal strongly resonate with the plane. If the cleric becomes truly powerful, he can become the "living avatar" of a plane and eventually merge with it while still alive, becoming the dominant personality of the plane in the process.
- Ordinarily, the souls of the dead float around in a special section of the Astral Plane for some time (a couple of decades, usually). They are then reincarnated into the Material Plane.
- To become a wizard, you must incorporate parts of various planes or demiplanes into your own soul during wizardly initiation. Normally, wizards avoid tying themselves to the bigger planes (unless they want to become mystic theurges or the like) and swallow parts of many different demiplanes and planes to make them less dependent on a single source. Casting arcane spells draws on the power of these planes, but unlike clerics, they don't ask for these powers, they take it. The planes and demiplanes tolerate this (and some even encourage this) because the rituals used during the spellcasting process increase their own power by small amounts.
- Incorporating parts of planes through wizardly initiation means that your soul won't enter the normal cycle of reincarnation when you die. Instead, you will become a demiplane upon your death whose size depends on your power in life. The typical apprentice will only become a tiny demiplane which will soon be absorbed by bigger demiplanes and plans, and whose intelligence will have little say in the larger compound intelligence. Archwizards, on the other hand, will become huge demiplanes, and should they become part of a compound intelligence plane, they have a good chance of becoming the dominant personality thanks to their power.
This is why most wizards are obsessed with becoming more powerful, and why they take such pains to prolong their lives...
- Sorcerors come into being when a fractured plane or demiplane becomes tangled up with their souls when they are reincarnated into their current bodies. They, too, will become demiplanes when they die, but unlike wizards, they did not choose to do this.
How does all this sound?
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