Because the seven rooms are symbolic of the stages of life, the placement of the great ebony clock in the final room that is black certainly signifies the role of the clock and the room in suggesting the progression of Time and Death, essential elements of of the finality of life which strikes fear in the hearts of the revelers.
As the guests and friends of Prince Prospero gather in the abbey where he has arranged a magnificent masked ball, they also mask their fears of the Red Death because the Prince has promised them security in this fortified "castellated abbey."
However, each time the massive ebony clock tolls the hour from the seventh room, all the revelers pause and the
...giddiest grew pale,...the more aged and sedate passed their hands over their brows as if in a confused reverie or meditation.
When the clock stops, the musicians resume their play and a light laughter in heard throughout the rooms. Yet, as the clock again tolls the next hour, there is the same "disconcert and tremulousness and meditation as before."
While the revelers again and again resume their gaiety, the seventh room remains empty:
there are now none of the maskers who venture; for the night is waning away; and there flows a ruddier light through the blood-colored panes; and the blackness of the sable drapery appalls...
Further, if a guest starts to enter the room, as his foot falls upon the dark carpet, the clock emits "a muffled peal more solemnly emphatic" as though the person approaches death.
When "a masked figure" enters this seventh chamber, the rumor of this personage spreads throughout the "whole company" as they experience first surprise, and then a feeling "of terror, of horror, and of disgust."
The figure was...shrouded...in the habiliments of the grave. The mask...resemble(s) the countenance of a stiffened corpse that the closest scrutiny must have had difficulty in detecting the cheat....his vesture was dabbled in blood...and...the face was besprinkled with the scarlet horror.
Prince Prospero, who has heretofore defied death and disease, becomes enraged by the appearance of this audacious interloper. "Who dares?" he demands as his words echo from the blue room. The figure of the grave and death moves toward the Prince, but no one attempts to touch him out of their terror. The Prince rushes toward the figure with a dagger in rage and in shame for his initial cowardice, but, again, no one of the terrified guests gives him aid. As he nears the macabre figure, who has turned back, this spectre now reverses himself to face his attacker; instantly, the Prince utters a "sharp cry" and his dagger falls to the dark carpet. Almost at the same time, the Prince himself topples and lies prostrate upon the carpet. Then, when the desperate guests rush the figure, they gasp in horror as this death-like creature's garments crumple in shapeless form upon the floor.
From this seventh room the Red Death's presence is now acknowledged, and the revelers drop, one-by-one in "the blood-bedewed halls of their revel.