Below the Demon Gods was the Demon Supreme and below the Demon Supreme were the Demon Kings. The entire Demon Realm was split into 666 Provinces.
Each Province was led by a Demon King, and they were ranked based on their average level of power. For example, Demon King Surtur's Province was ranked 453rd and was of average strength.
However the disappearance of a Demon King as well as an entire province was extremely strange and unheard of, especially with the events that preceded it.
There was an attack by humans and then a bunch of strange apocalypses that struck the entire province and then spread outward to attack other provinces.
By the time the Demon Kingdom made a response, 3 other provinces were almost destroyed. They managed to quell the power of the apocalypses with the help of a Demon God, who actually swallowed them as nourishment.