It is a poem that discusses the Christian judgment and tribulation. "The Day of Wrath" describes what will happen when the trumpet sound is made and God gathers the saved and unsaved. Dies Irae states the saved are taken to heaven and the unsaved are thrown into hell.
Originally it was performed by Catholics as part of the last rites service for the dead. There is also a popular version by Verdi, but the inclusion in Mozart’s requiem is also very well known. Mozart’s version is played in D minor.
This is also a piece of music that has a lot of controversy and intrigue associated with it. Mozart started writing it in 1791 but he died before this piece could be completed. In the end, the composition was completed by Franz Xaver Sussmayr. It is believed that in the original manuscript for requiem, the orchestra's intro, the Kyrie and Dies Irae were completed by Mozart. It is also believed Mozart composed the offertory.
Sussmayr is credited with the 'Agnus' and 'Sanctus' compositions contained within "Requiem". The whole piece consists of a variety of instruments such as the trombone, bassoon, trumpet, basset horn, violins, cello and organ.