When the striking of the great ebony clock announces midnight, the revelers halt and there is "an uneasy cessation of all things as before," signifying the guests' recognition of the force of time upon their lives. For, despite their masks and costumes intended to disguise them from death, they feel themselves vulnerable to the ravages of time in the announcement of that final hour.
In this very stylized short story, Poe develops the notion that art can hold an immutable and timeless beauty, subject to no disease or few other natural forces. For, by retreating to the magnificent abbey, "the creation of the prince's own eccentric yet august taste," Prospero and his guests, who don elaborate costumes, hope to escape death, and in point of fact, life itself. As one critic explains, by placing masks--additional faces, so to speak--they essay to escape life's terminableness by means of heavy cover over the essence of life held in by the skin; namely blood.
When the uninvited guest appears, he mocks this concept of art's infinity by wearing a costume himself. For this reason Prince Prospero's "brow reddened with rage, and he demands, "Who dares?" The enraged prince wants to know who has the effrontery to enter his abbey and who mocks his alternate world of artistic invention that he believes to be invulnerable to the contingencies of reality. However, what the prince, as well as his guests, have mitigated is the persistence of time. The striking of midnight by the clock sends tremors through all at the fortress of art as it serves as a reminder of this persistence and inevitable fate that Time brings to all people. Therefore, when the final hour strikes, the "masque of the Red Death" is revealed and the guests who have "seized the mummer"
...gasped in unutterable horror at finding the grave cerements and corpse-like mask...untenanted by any tangible form.
This "mummer" whose garb is "dabbled in blood," and whose facial features are all "besprinkled with the scarlet horror," signifies that life's essence spills from the deadly Red Death's presence in the final hour announced by Time's "gigantic ebony clock" despite Prince Prospero's and his guest's artistic efforts to hold it back.