The mysterious guest is an unnamed figure who descends upon Prospero’s masquerade at the stroke of midnight. He is depicted as a tall and gaunt entity who moves slowly and sadly through the rooms. When the partygoers take notice of him, he is dressed in a funeral shroud and wearing a mask, one which resembles a bloody corpse that has recently succumbed to the Red Death. His appearance angers Prospero, who feels that the mysterious guest's apparel mocks Prospero’s expensive and elaborate scheme to cheat the Red Death. Prospero chases the figure through all seven of the colored rooms, beginning in blue and culminating in black. Prospero drops dead as soon as he catches up to the mysterious guest in the black room and the rest of the estate’s inhabitants fall in a similar fashion when they, too, enter the black room.

The mysterious guest is the harbinger of death, destruction, and decay. But even more than that, he is the personification of the Red Death. In the text’s final scene, Prospero’s guests gather their courage and charge into the black room to confront the figure after Prospero suddenly dies. However, when the members of Prospero’s court take hold of the mysterious guest, they are horrified to discover that there is no body beneath the funeral garments and bloody mask. The lack of a corporeal form illustrates that the mysterious guest is not a person but an essence or an idea. The guest is not a regular partygoer; he is a reminder that death inevitably claims us all and it is foolish to deny it.