The Dark Abyss is a world of perpetual darkness. The Abyss was one of the first worlds visited by the deities and one of the few they actively avoided. All demons are tied to the world and are drawn back once killed.
All that is known of the Dark Abyss before the arrival of the deities is that it was once a fertile world filled with life before a cataclysmic event occurred, turning it into the hellscape it is now. In order to survive, the native races evolved into monstrous beasts that would be primal beyond most other lifeforms with a few descending into caverns to escape the more powerful ones.
When the deities arrived, they attempted to find a way to pacify the native population by creating a central figurehead that could corral the rest. However, their plan would backfire when the entity, now calling itself Devil King, rebelled and pushed them off its world. Centuries after their rebellion, demons reached a point of sentience and the title of Dark Lord is given to the leaders of their kind who consume the corpse of their predecessor.
At an unknown point in time, the demons and angels would come into conflict with each other, seeing the other as beings needed of extermination and a threat to their existence. Since their eternal world began, neither have gained any significant gains in their war.