https://infogalactic.com/info/Dies_Irae

“Dies Irae” (Day of Wrath) is a Latin hymn attributed to either Thomas of Celano of the Franciscan Order (1200 – c. 1265)[1] or to Latino Malabranca Orsini (†1294), lector at the Dominican studium at Santa Sabina, the forerunner of the Pontifical University of Saint Thomas Aquinas, Angelicum in Rome.[2] The hymn dates from at least the thirteenth century, though it is possible that it is much older, with some sources ascribing its origin to St. Gregory the Great (d. 604), St. Bernard of Clairvaux (1090-1153), or St. Bonaventure (1221-1274).[1]

It is a medieval Latin poem characterized by its accentual stress and its rhymed lines. The metre is trochaic. The poem describes the day of judgment, the last trumpet summoning souls before the throne of God, where the saved will be delivered and the unsaved cast into eternal flames.

The hymn is best known from its use as a sequence in the Roman Catholic Requiem Mass (Mass for the Dead or Funeral Mass).

The melody is one of the most-quoted in musical literature, appearing in the works of many diverse composers.

Catholic encyclopedia on topic: https://www.newadvent.org/cathen/04787a.htm