Beelzebub is mentioned in the both the Old and New Testament as the prince of the demons. He was also referred to by Jesus as the "master of the house."
Little is said about Beelzebub in the Old and New Testaments. He makes his first appearance in the Testament of Solomon, where he declares that he's a former leading angel. Solomon characterizes him as the cause of demon worship, the corruption of priests with lust, the rule of tyrants, jealousy and war. Beelzebub here may simply be another name for Lucifer. In the New Testament, he is mentioned in passing in Mark, Matthew and Luke. Most characterizations of him have come from fictional works after the fact. The name means "lord of the flies" in its original Ugaritic, and it may have stemmed from a Jewish belief that the fly was an unholy and demonic creature.
Beelzebub was popularized in fiction primarily by John Milton's "Paradise Lost" and John Bunyan's "The Pilgrim's Progress." In both stories, he is an antagonist and a high-ranking underling of Satan. In addition, Cotton Mather wrote a pamphlet entitled "Of Beelzebub and His Plot." In religious circles, he frequently has been considered a source of demonic possession.