The ebony clock adds to the plot because it is foreshadowing of death.
Foreshadowing is a hint by the author that something is going to happen. In the case of Poe’s gothic tale “The Masque of the Red Death,” the ebony clock foreshadows death. It is in the last room, it is black, and when it chimes everyone stops to listen.
The clock is described as standing “against the western wall, a gigantic clock of ebony” (enotes etext p 5). The clock is in the seventh room. In this room, almost everything is black or red. It is a very macabre room. It is dark and there are no lamps or candles, the only light comes from a fire and “the effect of the fire-light that streamed upon the dark hangings through the blood-tinted panes, was ghastly in the extreme” (p. 4). The clock is described in detail, and has a pendulum and a minute-hand, with a “sound which was clear and loud and deep and exceedingly musical.” (p. 5)
Although the sound is described as musical, it is also strange and disturbing.
“While the chimes of the clock yet rang, it was observed that the giddiest grew pale, and the more aged and sedate passed their hands over their brows as if in confused reverie or meditation.” (p. 5)
The fact that the clock is black, and that it is in dark red and black room, foreshadows the death of the party-goers. The fact that they all stop and listen, again foreshadowing their doom.
The clock strikes when Death arrives, and “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.” (p. 5) When everyone dies, the clock and the fire stop too.