What does the pendulum in "The Pit and the Pendulum" actually suggest? Also, what does an old man with a scythe represent?

The pendulum
itself represents two things: the passage of time and death. Pendulums are used in clocks to
determine and mark the passing of each second. As the pendulum swings back and forth, it is like
a countdown. That countdown leads to the narrator's death. This symbol of time is also the
instrument the leaders of the Spanish Inquisition have chosen to use to kill the
narrator. 

However, the narrator is so overwhelmed with fear that he actually
has no sense of time. Even though the pendulum swings indicate the passage of time, all he can
think of is the torturous wait. His sense of time is warped: 

What boots it to tell of the long, long hours of horror more than mortal, during which
I counted the rushing vibrations of the steel! Inch by inchline by linewith a descent only
appreciable at intervals that seemed agesdown and still down it came! Days passedit might have
been that many days passedere it swept so closely over me as to fan me with
its acrid breath. 

Just as he cannot stop time, the
pendulum is too heavy to stop with his own power. "Could I have broken the fastenings above
the elbow, I would have seized and attempted to arrest the pendulum. I might as well have
attempted to arrest an avalanche!" 

As he looks up, he sees the
personified Time (as a man) painted on the ceiling, but extending from this image of Time is the
pendulum itself. Death, personified, is often pictured carrying a scythe. When the narrator
looks up, it's as if he sees that image of Death: a man wielding a scythe (the pendulum). Again,
Time and death are conflated.