In Poe's "The Masque of the Red Death," the first, most obvious symbol seems to be the clock made of ebony, which is a black wood.
Ebony is described as...
a hard, heavy, durable wood, most highly prized when black, from various tropical …of southern India and Sri Lanka
It is important to realize that while the wood may be found in other colors (e.g., deep olive), we can assume this clock is black. (Black—in many cultures—is symbolic of death and/or mourning.) As the time passed, and...
...the hour was to be stricken, there came from the brazen lungs of the clock a sound which was clear and loud and deep and exceedingly musical...
The sounding of the clock is so loud and so unusual, that it makes all of those shut within the house pause, as they contemplate what they hear. For some, the sound is frightening (foreshadowing).
Another symbol is that of the masquerading figure:
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.
The costume this person has chosen to don is inappropriate—and the guests are appalled. This person's appearance closely resembles a dead body. It would appear that this person represents death. In fact, the figure seems to personify the Red Death—the disease that is obliterating the population outside the walls of the building.
Perhaps the third symbol is that of the building, but especially the gates, described at the start of the story, to where Prince Prospero "hides" with the others. They went into...
...the deep seclusion of one of his castellated abbeys.
It is an abbey constructed like a castle—according to the Prince's taste. Once everyone was inside:
A strong and lofty wall girdled it in. This wall had gates of iron. The courtiers...brought...massy hammers and welded the bolts. They resolved to leave means neither of ingress or egress...
The plan, then, is to keep everyone locked inside the walls of the abbey—but not just locked...the doors' bolts are welded shut. No one may enter or leave. It would seem that all possible measures have been taken to protect the occupants from the Red Death—and that nothing can harm the people.
However, with the appearance of the "mummer" in this particular costume, the Prince is enraged (and the others are terrified); he goes to stab the intruder...and falls to the floor dead. The guests realize there is no human being among the folds of the fallen costume—or the "mask." The guests now understand that this has been, in fact, a visit by the Red Death.
For all of Prospero's precautions, he has been unable to fend of the plague. The allegorical message seems to be that no matter how one tries, when his or her time has come, nothing can stop death. The clock has simply been counting down the time remaining for those about to die—the ticking of the minute hand has simply been measuring how little time in life remains for all those in this abbey.