When the Alpha looked over a newly formed Creation it became aware of a darkness within it that could not be explained - at least at first - eventually after much contemplation the Alpha realised that this "great shadow" was a part of itself, an embodiment of the Alpha's own shadow: knowing how dangerous this being was to Creation the Alpha engaged in a titanic battle with the entity - their battle would last for centuries and rocked the foundations of Creation, causing it to split into countless universes as a multiverse was formed from the shockwaves of the great battle between creator and destroyer.
Ultimately the Alpha stood victorious and cast the shadow into oblivion - deducting that while too dangerous to be left within Creation the entity could not be erased entirely either as it was a part of Alpha itself.
After this deed was done the Alpha retreated into the great beyond, deciding that it could now rest and allow the multiverse to grow without further interference from outside forces.
Conflict With The Gods
Centuries later the first of the gods were born, known as the Primal Gods, they were fierce yet loyal rulers of a cosmos still fairly young and as such they held power on a scale beyond those of their modern descendents - they safeguarded the ancient "forefather" species that would ultimately evolve into the countless array of life currently existing.
However the Primal Gods soon found themselves tampering in forces beyond even their own when they began to channel into a mysterious realm that lay "beyond Creation" - as they did so they were greeted by visions of destruction beyond anything they could imagine and in desperation they tried to break the link but it was too late.
Before the Primal Gods could react the great shadow was upon them, calling itself The Omega it declared it's intention to erase all Creation and began to spread across entire galaxies, the Primal Gods fought against The Omega and although powerless at first they decided to sacrifice themselves in order to save the other species of the cosmos.
The Omega absorbed all the Primal Gods energies as they died yet continued its path to an inhabited world known as Iode - their it appeared before the terrified Iodians as a cloud of night that bloated the sky, it spoke to them and said that it would spare them in respect for the sacrifices of the Primal Gods but that ultimately it would return.
Then as soon as it had appeared The Omega did leave, returning to the great oblivion and leaving the forefather species to survive in a cosmos no longer protected by the Primal Gods.
The Great SleepFor mileena following the conflict with the Primal Gods the entity known as The Omega went into a form of hibernation known as the "Great Sleep" in which it floated in the realm of oblivion, during this time The Omega was not truly asleep but rather connected to the cosmos in such a way that while never interfering the entity watched via "dreams" the evolution of countless universes, from the formation of planets to the rise and fall of entire civilisations across all of Creation.
The Omega is destined to remain in this state for longer than any human can imagine but eventually the entity will begin to stir, for like all beings The Omega can not "sleep" forever - eventually the great shadow will have to awake, though hopefully such a thing will not occur for quite sometime..
The Omega is a unique "Shadow Force" aspect that is considered the "Supreme Destroyer" - as such it is a Multiversal Constant that can induce Universal Collapse - erasing every living thing in said Universe as well as any connected realities, Abstracts or concepts short of God Force entities.
The Omega, despite its negative nature, is a tool for rebirth and any universe it destroys will be restored at a later point (usually by Alpha) - with the Omega acting as a "cosmic Flood", purging the old corruption to make way for new genesis and creation.
While completely invulnerable to all attacks, even those of Primordial Ones, the Omega can be stopped by God-Force entities or via a great sacrifice - upon recieving a suitable sacrifice the Omega disperses into a void, where it lays in a dormant state until awoken once more.