There was also Someone, who made that something outta nothing, and while He had a name He... well, it wasn't really a name, so much as a... y'know what? Let's just keep it simple and call Him Gawd.
So because all of that something was created of Gawd, it existed only for Gawd, and while that was good enough for awhile, even Gawd can get bored with everyone telling him how great He is. So He decided to create something that wasn't of Him. Y'know, for shits and giggles.
As good as He was at creation, He had no idea how to create anything that wasn't Him, so He did the next best thing - He hacked a piece off of Himself. You might be asking yourself, how does the Almighty cut himself? Well, first he created a knife. Then he made it real sharp. Then he cut off a big steamin' hunk of Himself.
Never having experienced pain before, he didn't know that doing something like that hurts like a sumbitch, but he figured it out real quick.
The chunk of godflesh fell through creation, leaking the stuff of creation, until it hit the very bottom, where reality was still hot and new. With a "clang", the Gawdknife fell right next to it. Now, this bit of godflesh was confused. It had just been a part of Gawd, and now it was alone. Squinting what passed for its eyes in the harsh light of creation, he could just about see the outline of Gawd waaaaay up at the top of it all. So this little orphaned bit of the Infinite started working its way back up there to figure out what Gawd wanted it to do. Spying the discarded knife, the cast-off grabbed it, figuring Gawd would want it back.
So it came to be that Gawd (whose thoughts had turned to other matters and promptly forgotten all about its discarded bit of Self after His brush with self mutilation), was horrified when it saw this mangled bit of Himself crawling up through the layers of His creation, a Knife which could harm Him clutched in its grasp, and he came to know Horror as he recoiled before his Creation.
Fearing that this bit of Stuff intended to do him harm, Gawd did what anyone would do in such a situation - He panicked. Throughout the All, entire worlds flooded, fire rained from the sky and firmaments cracked and heaved as Gawd threw up obstacle after obstacle to keep his Creation from reaching Him.
The newly sentient bit of Gawd, who for simplicity's sake, we'll call Saytin from here on out, was at first confused, then angry as his Creator destroyed entire realities, just to keep him away. Finally, he lost his grip on Jacob's Ladder and he Fell. At some point in the fall, the Godknife was lost.
As he tumbled, he reached out, grasping anything he could, and it was thus that what would come to be known as Demons were created, as he snatched beings from various realities, and they Fell along side Saytin.
Finally, they hit the bottom again, the fiery abyss from whence he had come, and it was there that he began to plot his revenge against his Maker. Since Gawd was all about Creation, Saytin would be destruction. Gawd wanted glorification, so Saytin would corrupt and turn them away from Him.
The battle was joined.
Over the millenia, the various layers of reality have been divided into two camps, with an ever-shrinking "Goldilocks Zone" of realities that are subject to the influence of both parties. Gawd has placed a bounty on Saytin's head, but he and his agents seem to slip through the Angelic Bounty Hunters' fingers every time. Battles are fought, plane by plane, world by world, until Saytin is either tossed out on his pointy ear, or he claims it for all eternity.
Waaaay down, at the very bottom of that No Man's Land between the two camps lies a patch of dry earth called Tayxis. Gawd wants it, Saytin almost owns it. Almost, but not quite. If Gawd can get His hands on it, He's got a beachhead into the Lower Levels - a perfect launching pad to reclaim what is His, and end the rebellious bit of His flesh, once and for all.
Little do both parties know, the Godknife landed Deep in the Heart of Tayxis, the most powerful weapon in creation, and all it wants is to taste the Flesh of the Almighty again. There are two sources of that flesh, though, and whose Flesh it tastes depends on who finds it first.