The Chthonic Gods

Chthonic Gods are the dark reflections of the Aesir Gods. Each Aesir has one, yet not all of them are equally problematic. 
[X] used to be the God of Death, Darkness and Despair. Under his reign, Aether Wraiths carried out the task of bringing mortal souls to [X] after death, where they were ripped apart and dissipated into nothingness, just as they had been before Veridian gave them life.
Aethereos, in his infinite quest for knowledge, grew frustrated at this practice, as with each mortal soul's destruction, their knowledge was destroyed with them. He needed a way to preserve the souls after death, and so hatched a plan. 
He made a pact with Thanatos, Chthonic god of Vanity and dark reflection of Veridian, god of life. Aethereos saw the strength in the potential of the new god of death being the dark reflection of the god of life, and thus made a prophecy to ensure it would come to pass.
This resulted in the war between Thanatos' army of Valkyries and [X]'s Aether Wraiths. Little is known of this war on the Central Plane, but some shrine of fallen Valkyries depict the battle, indicating it may have been fought across the realms.
The details of [X]'s demise are mysterious, but once defeated, Thanatos took his place, turning his aspect of Vanity into Beauty, and taking [X]'s domains of Death and Darkness.
Once the fighting was over, Aethereos helped Thanatos set up a new afterlife, gifting him the Sands of Time. This, of course, helped Aethereos' goals of preserving knowledge as much as it helped Thanatos. The Valkyries took over the Aether Wraith's task of bringing the mortal souls to the afterlife, and Aethereos tasked the Aether Wraiths with safeguarding the Sea of Stars instead.
Before this conflict, the life used to be truly finite, ending in great nothingness. This new afterlife became one of reflection on one's achievements in life. This was key for both deities, ensuring people live their fullest life before making their way to the Sands of Time.  
This resulted in a great age of enlightenment, with mortals ever-more committed to achieving great feats, resulting in the eventual ascesion of two elvish societies, and a long time of peace and growth.
Unfortunately however, things are now finally changing, as unbeknownst to all, [X] was not completely vanquished, and has been causing a great deal of the instability that has begun to shake the realms.