The Demon Lord is the title given to the king of Demons. The only named Demon Lord is Wilbert, who died a year ago. The name of the current Demon Lord is Leohart.
Origins
Nothing is known about the title's origins since the history of the Demons wasn't explained yet.
History
Wilbert was the first Demon Lord to stop the war between the Demons and the Heroes. However, he died one year ago and his powers were inherited to his only child, Mio Naruse. That's the reason why Leohart the new Demon Lord wants to get his hands on Mio's power.