I changed the sentences as you suggested to me. Thank you.

1) Looking up at the pendulum, he realizes that its movement has become wider and quicker and that it is coming down. Its lower part is of bright steel, about a foot long; the horns (what are they?) are upwards and the bottom is as sharp as that of a razor.
2) It hangs from a brass rod and it hisses as it swings through the air.
He believes the pit is the destiny the monks had prepared (??) for him. It is typical of hell and is considered the last instrument of their punishment.
3) The hungry rats eat the meat and the strap, freeing the protagonist. But he soon sees a light coming from the bottom of the wall that make (?) the figures spectral and unbearable, like demons looking at him from different directions.
4) Besides, the shape of the cell has changed. Two of its iron angles are acute – two, as a consequence, obtuse. The square is therefore turning into a lozenge that is growing flatter and flatter.
5) The horrors of Toledo come to his mind and he imagines to be the victim of the Inquisition.

1. comma after "upwards"

I have never heard the word "horns" associated with a pendulum before.
http://www.google.com/search?q=pendulum&hl=en&rlz=1C1GGGE_enUS379US379&prmd=ivnsl&source=lnms&tbm=isch&ei=1dpgTpXYDejnsQKx4Gw&sa=X&oi=mode_link&ct=mode&cd=2&ved=0CBkQ_AUoAQ&biw=1366&bih=643
None of the different images of pendula here have anything on them that could be called horns. Perhaps the author here is imagining a design that looks something like having two battle axes on this pendulum, but I'm not sure.
http://www.google.com/search?rlz=1C1GGGE_enUS379US379&sourceid=chrome&ie=UTF-8&q=battle+axe

2. "prepared" not "had prepared"

3. change "But" to "However," (including the comma)
"that make the figures seem spectral"
or
"that make the figures appear spectral"

4. change "Besides," to "In addition,"
(including the comma)

5. comma after "mind" -- add "himself" after "imagines"