Poe's main aim in "The Masque of the Red Death" is to create a growing sense of fear and dread in his reader, starting in the opening sentence and building straight through to the story's dark climax. There's nothing in the world Poe creates which lets the reader feel at home, no source of comfort or stability. The reader's fear is also mirrored in the fear felt by the masqueraders in the story towards their own death and anything that reminds them of it. Over the course of the story, their fear builds in a clearly traceable manner, from a nervous unease to an "unutterable horror" at the climax.
Prospero's revelers do not experience any genuine fear until the Red Death actually appears at the ball.