Perhaps Poe peruses the paradox that lies in humanity's affinity for chaos. The demon (I would probably argue him/her to be a/the symbol of chaos) and his/her attempts to sway the man on the rock with various forms of chaos--trumpeting elephants (behemoths), roaring hippos, torrential tempests--ultimately fail relative to their intent: they are meant to drive the man away, to wash away his composure and send him out of sight; but they do not. Only when chaos removes itself from view, only when all becomes silent does the man find himself unnerved, terrified by the nothingness and compelled to flee.