Beelzeblub flies around the room and periodically spawns either a larger Small Leech which charges at Isaac or a pair of Small Leeches that follow him.
Occasionally, Beelzebub jumps towards the player, releasing a chaotic burst of tears in all directions.
After a few Small Leeches are following him, he sends out a burst of 8 tears in all directions and makes the Small Leeches charge at Isaac.
Upon death, Beelzebub bursts into more tears and releases some leeches.
Beelzeblub is very similar in appearance and behavior to The Duke of Flies. Beelzeblub may be considered as a drowned version of the Duke.
Beelzeblub's name is a combination of "Beelzebub", a demon known as the Lord of the Flies, and "blub", an onomatopoeic sound that can be used for the act of trying to breathe underwater, referring to his drowned state.
Beelzeblub has an autopsy stitch. Again this can be seen on many other bosses as well. Including The Duke Of Flies.
In the Binding of Isaac repentance, Beelzeblub is known as Lil Blub