Also, I forgot originally to give a little more detail on this.
The first movement is based on the description of Cauchemar and his witness of the village below. It's a small prelude that is based in a 8th that is either on the two lowest D-sharps of the piano or the two immediately above those (or both), and a shift between D#, E, and D#. It starts with a few notes on bells, and then goes into a minute or so of solo piano before using the full (small) orchestra.
The second movement is a paraphrase on the Dies Iræ. The beginning stanza is repeated twice in the beginning, once with solo piano and once with all instruments, then there is a fugue in which the rest of the Dies Iræ is played on piano, in a bass, alto (two at the end), and soprano section, whilst the third stanza of the Dies Iræ is played on organ as a slow bass line. After, the fugue is repeated with all instruments, whilst the fourth stanza is played on organ as another bass line.
The final movement is a combination of the two, with the parts of the first stanza of the Dies Iræ paraphrase replacing the D#, E, D# shift in the first movement.
I did not describe this well, due to being distracted and tired.