"played the Dies Irae on a harmonium"

 'Dies Irae' means 'Day of Wrath' in Latin.  As a piece of music, it was part of the Requiem Mass used in the Catholic church, and other forms of Christianity which use Catholicism as a base.  It was originally written in the 13th Century, by Thomas of Celano.  Although generally not used now in the Requiem, it is still sung in a hymn and used in other missals.  In its original form, the musical setting follows the Gregorian tradition; it has, however, also been set to music by composers such as Mozart and Verdi.

Listen to Dies Irae (Gregorian).

Listen to Dies Irae (Verdi).

Listen to Dies Irae (Mozart).