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Derkila is the One Demon King of the Demon World. He is the most powerful and influential demon in society, however his whereabouts are unknown.
Sullivan said that the Demon Lord is the Absolute Supreme who dominates the demon world, "TOP OF TOP who manages 13 Crowns". The Demon King himself was the demon world itself, so Delkira's orders were absolute, no matter how unreasonable. If everyone decided to "Go to the right", but if Delkira said "Go straight" then even if there was no road, the demons would follow the Demon King's will, creating a path for themselves. And like that, rivers in the sky, flower fields on lava, such unbelievable things created the demon world. Iruma, after observing an incredibly beautiful demon world like today, commented that Delkira was a very interesting Demon King. 
According to Sullivan's recollection, Delkira has a very disdainful attitude towards Thirteen Dinner and 13 Crowns, calling them "idiots" ready to climb "the table surface" - the demon world that he created.
Little is known about Derkila, he was the Demon King a few hundred years ago, but mysteriously disappeared.
A few hundred years ago (before his disappearance), Sullivan was a subordinate serving Derkila, the relationship of both seemed very close.
Derkila have been in throne as Demon King presumably because of his highest Demon Rank, and have incredible levels of magical power.
Autokeep (Perfect Storage) A permeating magic that allows a certain place, or location to be kept in pristine condition, away from decaying, rust, dust and withering. This magic is currently being cast on Royal One's classroom in Babylus Demon School, even after being sealed many years.