Ril3y, yes Mozart and Verdi each included the Dies Irae within the Requiem Mass. The words were originally written as manuscripts as early as the 11-12th century, and music was sung not within the Mass itself, but at the intermit of the casket at the gravesite. It is no longer included within the Roman Catholic Requiem Mass and seldom sung other than within cloister roman catholic religious communities at burials. When sung in its original form the words are not accompanied by music. Today some monastic order required to maintain the tradition of the Gregorian Chant continue to sing the hymn at the gravesite, but to the best of my knowledge it is no longer included in any form in the rc Requiem Mass. I am fluent in reading latin, and reading Gregorian chant. The words of the dies irae have various (similar) translation and included in poetry, art, movies, and even in the original novel about the Phantom of the Opera. I find the hymn sung without music in the Gregorian style very sober, and terrifying. Ironically if you listen to the Gregorian chant and exclude the words of the dies irae it will sound familiar because it is often played as background for gloomy, scary, and horror movies. Orff's carmina burana also written from Latin manuscripts, is similarly used for background music for movies dealing with death, horror, . If you are really interested, while I don't usually refer a person to Wikipedia, I found the information very accurate and inclusive. I had forgotten how the original Latin manuscript influenced the arts, to include poetry, art. https://en.wikipedia.org/wiki/Dies_irae. Stay well, and thanks for bringing up the question, it took awhile for me to recall how sober the hymn sounds in chant, and how beautiful it is in other art forms. I dislike the theme of the hymn but it will outlive all of us. Smiles.