Dies Irae (the plainchant)

1. how you come across to it:
In history lesson, perhaps?
The music is quoted in Berlioz, in Liszt, in Rachmaninov, in Sondheim. The text is used in most requiem.

2. why this piece?
I were teaching some music students yesterday, and this seems to be a must-listen-to example.

3. (and...)
I've been overwhelmed by news which make me too angry these days. The words "Dies Irae" just keep hanging in my head.