The devil. ‘Beelzebub’ (= ‘Lord of Flies’), in AV, following Latin Vulgate; Greek MSS have Beelzebul (= ‘Lord of Heaven’ or ‘Lord of the House’), which is the more reliable spelling. It is a corruption of ‘Baal-Zebul’ and may refer to the god of Ekron (2 Kgs. 1: 2 f.). It is used insultingly by Pharisees (Matt. 12: 24) to persuade the crowd that the powers which astounded them were not Messianic but demonic.
Jesus' response is that demonic power does not effect exorcisms; Satan would hardly be engaged in committing suicide.
The argument is couched in the thought-forms of the 1st cent. and was doubtless persuasive. For modern people who attribute blindness and dumbness to an explainable physical defect and who do not recognize the god of Ekron, or Zeus, or any other prince of demons, the discussion has no basis in reality.