A/N So this first chapter is basically just to set up the story. The actual story begins in the next chapter, with OCs. This story is what Xehanort wishes he knew; it's about the events leading up to and including the great Keyblade War. Thanks to pretty much every Kingdom Hearts Wiki on the internet for having stuff compiled, such as an explanation of the realms in the KH universe. Anyway, here goes. And as a disclaimer, I don't own Kingdom Hearts, Square Enix, Final Fantasy, or Disney. I also don't own the Keyblade - and yes, of course, there will be Keyblade wielders.
Chapter 0: Scientific Background
Some scientists believe that Earth as we know it is different from its original incarnation. These scientists also believe that Mars was not the fourth planet in this solar system, but rather, the fifth rock from the Sun. The fourth planet existed in the space between Earth and Mars, and was in many ways similar to Mars. This planet, Theia, was the mother of our moon, and if this theory is true, then the impact from Theia being pulled into Earth's gravity and crashing through the planet would have drastically altered anything on the Earth at that time, likely even killing most life forms, as well as certainly destroying Theia.
One could hypothesize that, on the original Earth, and even potentially on Theia, life existed. One could even argue that humans could have existed on early Earth, then been wiped out by the collision from Theia. But this is where this story diverges from science and takes its own path.
What if Theia's crashing into the Earth was caused by something other than just gravity? Could it have been called forth by magic, destroying the world? What would happen to the world, in this scenario, instead of simple reforming?