In the beginning, Artos, God over all, split himself in two and bore two children. Atha, goddess of Light, and Nimorn, god of Darkness. Without guidance or guile, Atha and Nimorn wove the stars and Nebulae from themselves, giving freely in a symbiosis and duality of purpose.
Their first born twins, Cythia and Argessa, took their parents’ love of creation and bore upon themselves thriving ecologies. Because they share their love and ability to create life, each had to borrow, in turn, the other’s power to birth the ultimate of creations, intelligent life.
Atha and Nimorn next bore a son, Tril-sung. Tril-sung was a giant amongst the gods, dwarfing even his parents in sheer mass. For all his size and strength, Tril-sung was unable to grasp even the rudimentary aspects of creation which came so easily to his sisters.
Three more sons were born to Atha and Nimorn; Grul, Sath, and Peth. Grul was content to follow Tril-sung around and learn everything he could from his older brother. Sath, a dark, brooding type, remained out of reach of his siblings and kept mostly to himself. Peth, the last of the firstborn, swore to serve his elder sisters in any way he could so that he might one day learn the secrets of creation.
Sath came to Tril-sung in secret, declaring a desire to become closer to his powerful brother. He explained that he had a way to punish Cythia and Argessa for their greed in keeping the secrets of creation from the rest of the firstborn. He explained that if they were to work together, they could destroy Grul and use his power to harm the twins without their parents ever knowing. The Covenant of Darkness was begun.
When Nimorn would take council with Atha, Tril-sung would crush Grul between himself and Sath, then fling some of Grul’s power at the twins. Because Atha could not see through the darkness and Nimorn cannot see through the light, none would know who had done the deed. The plan went off without a hitch. That is until Peth, keeping his word to protect the sisters, used all of his might to steer the piece of Grul clear. The shot was close enough to weaken the sisters dramatically.
Since this time, only one of the sisters has the capability to sustain a thriving ecosystem at a time. Every few millennia, when Nimorn blocks the view of Atha and Tril-sung hides Sath, Cythia and Argessa exchange power.
After that, take a look at your wiki. There is some more helpful info there.