In the beginning there was one universe, and one immense, unifying oversoul, which would later come to be the demiurge. Some unknown period of time after the beginning a small part of the demiurge began to grow on its own, consuming the existing oversoul to fuel its on expansion, like a cannibalistic cancer. It grew exponentionally in power until the demiurge, unable to stop it, lashed out with the entirey of its power.
The ensuing clash purged the universe of sentient life, their souls snuffed out, and shattered the universe, splitting it into many others--not any smaller, but with the original universe's mass scattered among them. Each of these lesser universes carried with it a fragment of the soul that had grown from the demiurge, which then began growing again, on their own, each becoming the oversoul of its own universe. The demiurge itself had no universe of its own, left drifting between the fragments of what had once been its home.
That would be not be the end of it, for the scattered oversouls all still bore a connection to the demiurge, and continued trying to feed off of it; each was tied to a difference aspect of the primordial soul, which to save itself purged those aspects from its nature, one at a time, nothing was left but a darkness blacker than the void and a lonely, embittered will.
One of those oversouls, however, fed on something the demiurge could not rid itself of: either its consciousness, or its very essence--its 'soul', though technicall the demiurge itself was nothing but a single, immensely powerful soul, weakened though it had become.