The Demon God King, the aggregate form of Solomon's seventy-two Daemons. To begin with, the 72 Demons were "a system to efficiently promote proper reason" invented by the founder of (human) Magecraft, Solomon.
SpoilerGoetia was originally created as a magical ritual to look after mankind after Solomon's death, the Human Order Correction Ritual. Said system grieved and raged against the relationship between King Solomon and humans, becoming a curse that takes action to conquer “the imperfectness of humans” that King Solomon continued to ignore. It eventually changed into the Human Order Incineration Ritual. A magecraft with its own will that was devised in order to attain true wisdom, and is now attempting to reach the origin while making use of the entire human history.
After King Solomon’s death, the 72 demons that were sealed away inside his corpse awakened to their own ideology, swindled Solomon’s name and embarked on a new venture to convert the entire mankind into energy.