Beelzebub

The name "Beelzebub" was one of the earliest words Mary Minett found in her father's dictionary. She thought it to be a beautiful name until she learned that it was derived from the Hebrew phrase "ba 'al zebûb", which meant "Lord of the Flies. (PROSE: Casualties of War)

According to Olive Hawthorne, "Beelzebub" was one of the names of the Devil, alongside "Satan" and "Lucifer". As such it was among the names by which she called Azal while under the impression that he was the actual Devil, as opposed to a Dæmon whose image had influenced popular illustrations of the Devil. (TV: The Dæmons)

Behind the scenes
In the real world, opinions vary, both in theology and in fiction, on whether "Beelzebub" is the same entity as the Devil or a different, lesser demon.

In the 2019 TV series Good Omens (starring David Tennant and Michael Sheen), the character of Beelzebub was played by Anna Maxwell Martin, who appeared in Doctor Who in The Long Game.