What is the masked visitor?
As an allegory, Poe's "The Masque of the Red Death" can be interpreted as a tale of the folly of humans when faced with their own inevitable deaths. For, death is the one guest that cannot be banned; no bars will keep him out.
Poe writes that there is "an assembly of phantasms," so death could easily have slipped into any of the rooms. In fact, the guests have heard his step before--"with the same solemn and measured step wich had distinguished him from the first" the narrator comments on the appearance of death for Prospero. When a "tall and gaunt figure shrouded from head to foot in the habiliments of the grave" materializes, his mask resembles that of a stiffened corpse (a death mask like those made for famous men afterwards); he shudders as in a death throe as the Prince, who stands in the blue chamber (death), demands "hoarsely," "Who dares?....Seize him and unmask him...." as he dies.
Now, since the narrative continues after the Prince has died, the Red Death must be the narrator, whom the guests realize has been present all along. For, he has not come as a masked visitor; rather, he has been in all the rooms as a "phantasm"; waiting for the Prince, the Red Death sends Prospero's death to him, wearing the shroud and death mask of the prince who stands in the blue room of death. Thus, the masked visitor is no visitor at all; he is merely the dead Prince Prospero who wears the death masque of the red death on his visage.