Baalzebub, &, is one of the demon princes who appears in Gehennom. He will be found in his lair, if you do not encounter him earlier. He is quite easy to defeat. Deal with him as you would any other demon. If you have enough gold, and if you are not wielding Excalibur, Baalzebub will ask you for money for safe passage. If he fails to see where you are he will attack you out of general principle.