Beelzebub, also known as "Lord of the Flies," is a scary and mean demon from stories and legends. People used to believe that he was a powerful and evil spirit who worked with the devil to harm people and make them do bad things.
Think of Beelzebub like a big bully who likes to cause trouble and scare people just to make them feel bad. He would try to convince people to do bad things, like stealing or hurting others, so that they would be in trouble with God. In some stories, it's said that he could even possess people - that means he could take control of their body and make them act differently than they normally would.
But don't worry, Beelzebub is just a made-up character from old stories and legends. He's not real, and no one can actually hurt you like he would. People used to use the name "Beelzebub" to describe things they thought were very scary or bad, but now we know that it's just a story.