The Demiurge is a complex entity. Billions of years old, it is a space-dwelling matrix encompassing dozens of parsecs. It is composed entirely of flows of magical power that had attained sufficient mass and complexity of connections to become sentient.
The structure of the Demiurge is called Yggdrasil by Asgard. These ways and paths bind together the multitude of demesnes where the god-races dwell. No realm is closer to the roots of the Demiurge than Earth itself; a planet teeming with mortal awareness and spiritual life, and at the heart of which is the ideologue known as the Life Entity.
The origins of the Demiurge reach back to the breaking of the First Firmament. A creature known as the Life Entity-- a primal force in and of itself-- was carried to safety by the Mother of All, the ideologue known as Gaea. She cocooned it in the center of the Earth billions of years ago and created a spiritual garden to nurture it. She was joined by other potent ideologues, including The Lady and the puissant Night. From Night's robes flowed the Infinite River, carrying limitless magic and inchoate potential. These flows assembled themselves into a network of sufficient complexity to create a low, dim awareness-- pre-sapient, only verging on the edge of consciousness for billions of years.
The First Gods, refugees of an ancient war, found the Demiurge hundreds of millions of years before humans evolved. They resettled and formed a home inside the Demiurge's matrix, feeding off the Demiurge's energy and reinforcing their new habitat. They came to call their new community Elder Gods.
Gaea-- a benevolent goddess with a maternal nature-- nursed several progeny into existence with the aid of the Demiurge. A corruption set in with the glut of resources and magical energies. The Elder Gods began warring with one another. They raised up civilizations and crashed them into one another. Creatures of lesser puissance slipped up through Hell and even tore doorways in from Nullspace. Gaea knew that if the Demiurge was corrupted enough, the nightmare legions of the Old Gods would crack reality wide and pull the entirety of the universe into the infinite entropic pit.
Gaea communed with her allies among the still-sane gods, who wished to see the universe preserved. A weapon was needed. Her son Atum was born a fully formed warrior. In his hands he bore a weapon forged with the mortal imperative of Destruction itself. The slaughter was cataclysmic and without mercy. Godlings, demigods, usurpers and ideologues of primal and crucial concepts were destroyed so utterly that they were excised from the collective memory of mortals and rendered into nothing but astral motes, returning to the Source. A few slipped through Atum's grasp, disappearing into the many dimensional planes coterminate with reality and located so close to the Demiurge's matrix.
The Raising of the Demiurge
Fifty thousand years ago, the Oans visited the Demiurge, hunting for a magical weapon to fight the aggressive Kryptonian Hegemony. A bargain was struck with the Elder Gods, and Oa assisted them in establishing the last critical junctures in the Demiurge's body. With a final push of effort, the Demiurge became wholly sentient, an entity of living magical force millions of light years across.
The surviving Elder Gods would spawn a new wave of deities, called the New Gods. They populated the empty dimensions adjacent to the Demiurge, creating worlds from nothingness. The differences between the realms were both gross and subtle; each new world added a new note of harmony to the Demiurge, attracting spirits from the Astral plane. Of the new realms, Asgard volunteered to repay the debt owed to Oa, and armed with magical weapons the Asgardian legions launched themselves at Krypton.
The rapid growth of power from the worlds of the Demiurge prompted the attention of the Celestials. Fearing the power of the Demiurge and seeing the potential that the New Gods possessed, the Celestials launched a war host to attack the Demiurge in 15,000 BCE. The war with the Celestials was staggering in scope, enlisted the armies of Atlantis, the New Gods and even the Elder Gods dwelling in the space. Millions died, and many gods were destroyed and harmony destabilized in their realms. Several pantheons were destroyed entirely. The Celestials neared Earth and prepared to fire a superweapon-- a singularity pulse that would have collapsed the entire galactic sector into a black hole. The threat to Earth prompted action from the Vishanti, who alerted the Demiurge to action. The Demiurge flooded itself with the Power Cosmic and burned out the 'infection' of Celestials.
The counterstroke against the Celestials sapped the flow of magic across the Demiurge and the safe routes of travel between the dimensions. The great betrayal of Morgan le Fay broke the Demiurge's back, as she summoned the puissant entity known as C'thulu to Atlantis. Binding the ancient monster to the floor of the ocean along with the city of R'yleh took all the power the Demiurge had remaining as it burned itself out to contain this threat. Magic on Earth trickled to nearly nothing and the civilization of Atlantis was obliterated overnight. Many of the more distant realms associated with the Demiurge were unable to ford the dimensional barriers between worlds, and were severely depowered with the absence of that entity. The Demiurge lapsed into pre-sentience once more, returning to mere dreams of what could be.
Thousands of years would pass before the Demiurge would be restored. By then, humanity had exited the Hyborean Era and entered the Age of Mythology. Worship of the New Gods resumed and the connections between realms were made whole. The Demiurge lurched back into sentience, once more a complete entity.