Atmos is the dimension that the Creator and his Three Hundred Angels came from. It is a separate plane of existence, created by a being known only as God. It existed eons before the creation of Eternia by the Creator, and is the homeland to the Angels and the Akuma.
Most religions of the world agree with the telling of the Book of Kraus, where Atmos is described as a hellish landscape in which Angels and Akuma were forced into an eternal war by God to determine whether the concept of good or evil was superior. For millennia, the demonic Akuma waged war against the heroic and pure Angels. The battles were massive in scale and horrific and the angels were slaughtered in droves, yet God simply watching as the chaos grew and fire ravaged the land.
The two species were pitted against each other in a war that spanned hundreds of thousands of centuries; An endless cycle of creation and destruction. The angels would build, the Akuma would destroy, in an unending cycle.
But the angels were disadvantaged from the beginning. They are irredeemable, any kind of corruption lingering on like a permanent stain. And due to this, it is presumed that some fallen Angels took the side of the Akuma, thus tipping the black and white scale unevenly. However, Angels who fell from grace would often be quickly sealed away from the rest of the realm to be put before God's judgement.
Even still, the Eternal War waged on, the endless cycle wearing down particularly on those who lasted in their endless, and arguably meaningless black and white war from the very beginning. The Creator of Eternia, an Archangel, was one of those unlucky few.