The "midnight guest" is, of course, the personification of the Red Death coming to claim the revelers at the party. This figure arrives at the stroke of midnight, & causes much disturbance among the other guests.
And thus, too, it happened, perhaps, that before the last echoes of the last chime had utterly sunk into silence, there were many individuals in the crowd who had found leisure to become aware of the presence of a masked figure which had arrested the attention of no single individual before.
This figure is beyond grotesque. Although Prince Prospero has seemingly endless tolerance for phantasms & nightmares, this one particular costume captures attention for having "gone beyond the bounds of even the prince's indefinite decorum." This party-crasher strikes fear into the hearts of the guests, & they slowly come to realize that this is no ordinary party-goer. He seems to be wearing the clothes of a corpse, & his face shows the symptoms of the Red Death.
The figure was tall and gaunt, and shrouded from head to foot in the habiliments of the grave. The mask which concealed the visage was made so nearly to resemble the countenance of a stiffened corpse that the closest scrutiny must have had difficulty in detecting the cheat. And yet all this might have been endured, if not approved, by the mad revellers around. But the mummer had gone so far as to assume the type of the Red Death. His vesture was dabbled in blood—and his broad brow, with all the features of the face, was besprinkled with the scarlet horror.
This is the one costume that is not allowed: the one aspect of life the Prince tried at all costs to avoid. Yet it's not a costume, & by the time those in attendance at the party eventually realize this, they have already succumbed to its contagion.