Interestingly, seven is a Biblical number used often because it represents completion and fullness, so nothing more can be added. In the Old Testament, this number is used repeatedly, beginning with the creation of the world as it took God seven days; likewise, in ancient Israel, the people also rested on the seventh day. Among the Deadly Sins, there are seven because this number covers all.
Thus, the seventh room of Prince Prospero's abbey-fortress represents the final room, the room of fullness and completion. The courtiers who engage in revelries in the abbey are reluctant to enter the final room because, like Shakespeare's stages of man, it represents death with its "blood-colored panes," the "blackness of the sable drapery" and carpet which "appalls" them, and an ebony clock which tolls on the hour, reminding them of the passing of time.
Of course, the fears of the guests are certainly warranted as the spectre who is shrouded in the "habiliments of the grave" that are "dabbled in blood" appears from this seventh room. After the Red Death claims all its victims, the fames of the tripods extinguish and the
life of the ebony clock went out with that of the last of the gay....And Darkness and Decay and the Red Death held illimitable dominion over all.
Indeed, the premonition of the guests about the seventh room becomes complete.