Some notes on the cosmology (how the universe is shaped) for my fantasy stories. This is consistent background for all of the fantasy stories I intend on ever writing.
In The Beginning... it was a nice day (sorry, Mr. Gaiman and Mr. Pratchett, I'll cease and desist right now).
In the beginning, there was one world. Universe. Call it what you like. For the sake of less typing, I'll say world. No word on how this world came into being; for all intents and purposes, it simply did, and that's good enough for me. Our own physicists can't say everything about why things changed to produce the universe As We Know It, so what chance would a bunch of mages, alchemists, philosophers, and holy men and women have in divining out the ultimate source of the whole cosmos? Magic doesn't even follow the same rules from world to world. Good luck being able to figure out how it all started...
So, one world. And then, by some unknown process, it... started to bud. Kind of like how a tree branch starts growing flowers that'll turn into fruit. A new universe started to bud off of the old one. Not in the "every decision creates an alternate universe" way, but in the literal "one universe gave birth to another" way. Think of a cell undergoing mitosis. When this new world was finished growing, it detached and became a true world of its own. Very, very similar to the first world, but there were substantial differences. Sure, the humans were still human, but how human is human?
Anyway, these two worlds separated, and each began to grow a new world of their own. Soon, in relative terms, there were four worlds. Then eight. And then sixteen. And so on and so on. Some old ones died, some new ones were the planar equivalent of stillborn, and others were freaks. Universes that couldn't support life, not as any human knows it. Too much of this, too little of that...
Which brings me to my fantasy stories.
It's pretty much going to be a given that in any story I so far intend on writing, this will crop up. These worlds are all damaged somehow. Some are worlds that didn't grow right, some are having problems because their "child" world isn't growing right. Sometimes the child world won't detach, and this threatens the integrity of both worlds, as they risk tearing each other apart. Or the stresses manifest in more subtle ways. Whole species might be born caught in both worlds, each and every member heir to endless pain from being stretched across distances that matter, energy, and thought should not cross. If there's a bogeyman -- not just a run-of-the-mill monster, but something arcane and profane and terrifying -- for the people of the world, you can bet they come from the other world. The olfen, mentioned briefly in a previous entry, are just such beings. They want into the protagonist's world because their world is effectively dying in the womb, and they need someplace more real, even if they're somewhat alien to it and its inhabitants.