The demons a a race that existed before Kirai. They were led from the shadows by the King of Hell until he arrived. The demons at that time were highly organised soldiers with a ranking system. The demons used the number of True eyes to determine an individual's worth
1 eye. A common demon. Just about strong enough to possess a doll, house, vase e.t.c.
2 eyes. Demon commanders. They can cause major breakdowns on a national scale, but can still be slain by a strong adventurer.
3 eyes. Demon Lords. Their presence puts the world at risk. It usually takes armies of adventurers to take one down, not to take about it's demons.
4 eyes. Demon Gods. They are demons that can crush worlds though it takes roughly 1 year. They are known for lower god-killing
5 eyes. Demon Elder Gods. Only a few exist. These beings can end 5 solar systems in less than 3 months. They are known elder god killiers
6 eyes. King of Hell. The strongest demon acknowledged by the demon races. He can end 70 solar systems in 10 days. At least that's his record.
7 and above are thought to be non-existent, except for Don'yoku who became a 20 eyed demon after drinking a drop of Kirai's blood.