Beelzebub (ベルゼバブ Beruzebabu) is the sixth boss from Ghouls 'n Ghosts. His name comes from the Philistine deity, Baʿal Zəvûv, which in Hebrew translates to "Lord of the Flies" and was adopted in Abrahamic religions as a demon. He attacks Arthur in the form of a giant fly, also appearing as a large swarm of insects when he moves, and Arthur cannot harm the enemy when he does this. Beelzebub is found in Lucifer's castle entrance as the final boss and again inside the castle before fighting Lucifer himself.
In demonology (the study of demons), Beelzebub is considered at the top of the demon hierarchy along with both Astaroth and Lucifer as part of the "evil trinity". They are the 3 boss characters fought at the end, though presumably both him and Astaroth are at the behest of Lucifer, who is considerably more powerful.