So. Heres the backstory for the setting: A Wizard Did It.
Heres the long version:
One day, a nameless wizard realized he didn't have much time left, and he still didn't have a magnum opus. So he used what he had. His library, his magic, and a walnut. He crammed all his magic, and all his media into the walnut. Then he shot it into space.
It found a nice demiplane and took root. From there, the very very long spell began. First it formed a library. That library defined the world around it, and was in turn defined by the world around it. Then it grew a grand tree. From there a few civilizations were seeded by the medias shoved into the walnut.
This library's most important book is its World Book. A book that is the story of itself. The library has reference biographies of every living being in the demiplane.
The library grew attendants. People who could never leave the library, who helped maintain, audit, edit, and moderate the World Book and the biographies. Anything that is in a biography or the world book is true. Sometimes, there is discrepancies. The attendants confirm the correct truth and plant it into the world book as needed. This happens very often, and is the core of the world's most primal magic. Blank Pages.
Any time the world encounters a cause it has no effect for, it spawns a blank page. For an incredibly long time, people did not know what these blank pages were. The blank pages hold the effect of the cause. There is a desert full of them. Incredibly powerful blank pages, with no known causes that attach to them.
There are a few magics on top of that, which are far more stable.
Everything has a spirit. Plants, forests, cars, bees, people, elves. You name it, it has a spirit. These spirits can be befriended or enslaved, and many of them inherently have power over supernatural things, metaphysical concepts, and natural elements. Befriending them is a lot more useful than enslavement. People who use these spirits for magical abilities are known as Binders.
When anything alive sheds its mortal coil, it becomes a spirit. Spirits live alongside reality, on a seperate "layer". Some are able to see between these layers. Areas have thinner or thicker boundaries between these layers.
Theres a huge font of raw magic from the library, similar to the blank pages, pouring out from that desert full of blank pages. It fills the wind. Wizards make use of these enchanted winds. They cause confusion in the transmission to the library. Using magic with these winds literally confuses reality. Strong mastery over a single skill can give one power similar to use of these winds as well. Its very likely these winds might be how the library receives its information, and how it can modify its demiplane in turn.
There are the magic rocks. Artificer's domain. The strictest form of magic in the walnut. There are many rocks all over the demiplane that are filled with elemental abilities. Red ones have the heat of flame. Green ones have the push of wind. Blue ones are fonts of water. Their effects can be combined to make powerful tools. Toss a red rock into a bucket of water and it'll boil. Supposedly, theres a way to turn them into powders and gain even more magical effects. No one has managed to do this yet, as any time you smash one all of its contained power violently leaves at once.
Finally, is probably the most powerful form of magic in the world. Rumors. There is a third "layer" known as the layer of Myth and Rumor. In the library, it might be seen as the writer's table of the attendants deliberating with one another. As rumors grow, they slowly become fact. A public conciousness. Eventually, they're indiscernable from reality. They're so well defined the attendants are forced to write them into the world story as fact. After that? They transfer from the Layer of Myth and Rumor, and join the Layer of Mortals, or the Layer of Spirits, as appropriate.
There are many many different types of communities in this plane. The above is the only constant within this plane.
Addendum: Several years later, another wizard did the same experiment with a very different library, and a chestnut instead of a walnut.