Another demon from hell, Beelzebub is perhaps one of the most famous as his name has become synonymous with Satan, even though the two are actually different creatures. One of the seven princes of Hell in Judeo-Christian lore, he was original a Philistine god before being taken over and made into a demon, as happened to so many other ancient gods (see also: Adrammelech).
Known as the Lord of the Flies, Beelzebub was often depicted as a large, fly-like demon. Although he does retain the flies in his appearance in Castlevania, his depiction as a man, cut into pieces and hanging from chains, likely comes from Beelzebub being references as one of the fallen angels who aligned with Lucifer against God. Being cast into Hell, for torturing as well as to serve the lord of Hell, likely is what the Castlevania series drew on.
Whatever the case, it is certainly true that Beelzebub, a beast that you attack until he slowly falls apart, losing limb as blood and viscera spray off, makes for one of the most disturbing bosses in the series, as well as one of its most memorable.
Fighting Against Beelzebub
Appearing, appropriately enough, in the Necromancy Laboratory in the Inverted Castle of Castlevania: Symphony of the Night, Beelzebub is already hanging from chains when hero Alucard comes into the boss chamber, immediately to be greeted by the buzzing of flies. To defeat the beast, Alucard has to attack body parts hanging from chains, most specifically the head (as that controls the rest of the beast), destroying the demon while he hangs there. Not that he's unable to strike back as Beelzebub, the Lord of the Flies, has at his command an army of flies to command. These flies will fly in large swoops around the room, damaging Alucard mightily if he gets caught in their wake. The Lord of Flies can also drop poisonous maggots that Alucard will have to avoid (or kill) to keep himself safe).
Although similar in style and appearance for his second run in the series, Beelzebub has a few new tricks come Castlevania: Harmony of Despair. His fight actually ends up going through a couple of phases, with the first phase aligning with his original appearance in Symphony. Once he's been damaged enough, though, the flies he summons will start to move in patterns, and not just in a continuous wave, forcing the player to learn the various patterns of horizontal lines and find ways to avoid them while still laying on the damage. This battle is more frenetic, and feels more difficult, so the player needs to stay on their toes (especially in Hard Mode) if they want the glory of winning the day.