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Montag is drifting down the river, trying to escape the mechanical hounds and basically society that is chasing him. The "great, black creature" is the land he spots from the river at night. It seems like an endless mass which he no longer is part of. The dark shadow is like his life past which he can't return to.It is a literal personification of the darkness which his life always has been. He feels alienated and alone drifting somewhere in the darkness.