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 magical power that had attained sufficient mass and complexity to verge on self-awareness.
The origins of the Demiurge are lost to dim history. A creature known as the Life Entity-- a primal force in and of itself-- cocooned itself under Earth billions of years ago. The existence of the Life entity attracted flows of magic from The Source at the heart of the universe. Those flows assembled into patterns roughly analagous to a brain, creating a low, dim intelligence.
The First Gods, refugees of an ancient war, would find the Demiurge 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, would nurse several progeny into existence with the aid of the Demiurge. Grown fat and indolent, several of the Elder Gods began warring with one another. Gaea's son Atum was born a fully formed warrior, and at her behest slew his corrupted relatives. Several slipped through his grasp, disappearing into the many dimensional planes coterminate with reality and located so close to the Demiurge's matrix.
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 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.
This act 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.
Thousands of years would pass before the Demiurge would be restored. By then, humanity had exited the Hyborean Age and entered the Classical Eras. 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.