I'm The Strongest Demon Lord but nobody is coming to slay me so I decided to infiltrate heroes

Lucifer, the world's most powerful demon king, ruled the Demon Nation with its overwhelming power. I also made a harem where I exchanged my beloved wife, and my life lasts for more than hundreds of years just eating, fucking and sleeping. But the destructive urge to sleep in the blood of the Demon King caused me pain in search of a mighty enemy. Even though we wait so long, why aren't the brave men coming to crusade themselves is a real tease! Lucifer, who thought so, decided to turn into a human being and go see how the military school, a brave training institution, was doing. The best way to know what's going on inside is to enroll. Lucifer successfully enrolled after a minor breakthrough in the admissions exam......!? This is the story of what the mightiest demon king has done wrong becoming the supreme brave man.