The Abyss had existed for millions of years. No one knew where it came from, other than that it craves destruction and would do everything in its power to kill and devour souls so they could assimilate them in their ranks.
For so long, the rankings of Demons had been arbitrary, only assigning them concretely after the start of the Cosmic War against the Abyss.
Rank-1 till 6, and from there, would be Rulers, Kings, and Emperors. But, there were also the rare cases where the Abyssal Demons grow so strong from devouring its kind and living souls that it reaches the rank that needs ten or more Emperor-Class Cultivators for it to be destroyed — a God-Class Demon.
The last appearance of a God-Class Demon was when the Abyss attempted to take over the Origin World. There were ten God-Class Demons, and when this happened, it was in the midst of the Spiritual Upheaval that led to billions of dead.
Only a handful of weakening cultivators remained.