Not that I think Milton was concerned with plot efficiency while writing Paradise Lost, but the inclusion of the allegorical/not allegorical shape of death feels a little strange. What is its purpose within the story? It would be one thing if it was mentioned briefly as part of the setting and then forgotten like the hell hounds or Scylla, but Death becomes its own character with its own motivations that are opposed to Satan’s. Death’s presence in the story becomes more mysterious when you consider why it might want to stop Satan from passing through the gates of hell. Death tells Satan “Back to thy punishment,” so perhaps it is on the side of God? Then there is Death being described as a Goblin in addition to its description as a formless shape… Maybe it is best not to think about these things through an entirely rational frame of mind.