The Conflict
Prelude
In the beginning of the universe, Evelyn and Erebos were the first gods to create life, having volunteered. Evelyn created the angels, beings of morality and justice, and Erebos created the devils, creatures of earthly desire and freedom. These two races were meant to balance, and learn from, each other. While for a short while, this seemed to be the case, things quickly became heated between the angels and the devils. The angels were disgusted by devils' "depravity" and their eating habits (devils feed on sexual, emotional, or generic energy.) Devils, in turn, grew to dislike the angels' constant preaching and condescension. As tensions rose, Evelyn and Erebos tried to intervene, but ultimately began arguing with each other on whose children were right, only succeeding in escalating the situation. Finally, as tensions reached their peak, Evelyn and Erebos declared war on each other, descending to the mortal world.
When Evelyn and Erebos descended, they took charge of their respective creations, each becoming the commander of their own army. Angels and devils were sent every which way, covering and taking as much territory for themselves as they could.
Not long after Evelyn and Erebos created their "champions", the Seven Virtues and Seven Sins, respectively,
Eris decided he had seen more than enough. With no mercy, Eris descended to the battlefield, wiping out millions of angels, including all fourteen Sins and Virtues, much to the grief of Erebos and Evelyn. Both sides were soundly beaten and almost routed. In his anger, Eris demanded that angels and devils no longer be allowed to live in the mortal realm.
In response to Eris's demand, Evelyn and Erebos created Heaven and Hell; Heaven to protect the angels from temptation and sin, Hell to free the devils from judgement and disgust. Neither race had been given any reproductive ability; in the first place, Evelyn and Erebos could not foresee such a great loss of life. Eris suggested that Evelyn and Erebos simply give the races the ability to reproduce, but to them, it was the same as admitting defeat to each other. Instead, all previous angels were demoted and transformed into "cherubs", child versions of themselves. Likewise, Erebos transformed every devil present into an imp. Both these changes could be reversed by finding a large amount of their respective energies in Heaven and Hell; light energy and dark energy. Additionally, Erebos and Evelyn no longer speak to each other.
Historical Significance
No other races were alive to witness the Light-Dark War, but many stories and theories have been discussed about just why angels and devils live where they live. The most commonly agreed upon, and incorrect, theory, is that it has simply always been this way, since the dawn of time itself.