History and theological purpose of the Sequence portion of the Catholic Requiem Mass liturgy

In [Mozart's Requiem](https://en.wikipedia.org/wiki/Requiem_(Mozart)) (K. 626), the most musically well known (apart from the Introitus) is the [Sequentia portion](https://en.wikipedia.org/wiki/Requiem_(Mozart)#III._Sequentia) consisting of the following sections: - Dies irae ("Day of Wrath"), [8:06](https://www.youtube.com/watch?v=54h8TxJyNy0&t=486s) - Tuba mirum ("Hark, the trumpet"), [9:47](https://www.youtube.com/watch?v=54h8TxJyNy0&t=587s) - Rex tremendae ("King of tremendous majesty"), [13:31](https://www.youtube.com/watch?v=54h8TxJyNy0&t=811s) - Recordare ("Remember"), [15:51](https://www.youtube.com/watch?v=54h8TxJyNy0&t=951s) - Confutatis ("From the accursed"), [21:31](https://www.youtube.com/watch?v=54h8TxJyNy0&t=1291s) - Lacrymosa ("This tearful day"), [23:50](https://www.youtube.com/watch?v=54h8TxJyNy0&t=1430s) (links above are to a [sample performance with English subtitles](https://www.youtube.com/watch?v=54h8TxJyNy0) by the Warsaw Philharmonic Choir and the Polish Sinfonia Iuventus Orchestra, 2019) Reading the text, it's obvious that the purpose of the *Sequentia* sections are to provide various reflections on particular judgment, general day of judgment, resurrection of the body, atonement of Jesus Christ, and petition for mercy by Christians after death. Mozart's beautiful musical dramatization of each section (written on the year of his death, 1791), makes his Requiem setting especially popular for concert music and recordings to this day. **My question is**: what is the origin of the Sequentia portion of the Catholic funeral mass and the function of it within the liturgy? Does it replace the Credo? Who wrote the text? Is it still part of the funeral liturgy today? **Helpful resources**: - Latin/English [translation](http://www.phoenixsymphonychorus.org/uploads/7/3/5/4/73548593/mozart_requiem_pronunciation_and_translation.pdf) of Mozart's Requiem - Brief musical introduction and analysis from *The Classic Review* by David A. McConnell (2020): [Mozart - Requiem - A Beginner's Guide](https://theclassicreview.com/beginners-guides/mozart-requiem-a-beginners-guide/) - [An article about *Dies Irae* Sequentia](http://chantblog.blogspot.com/2009/06/missa-pro-defunctis-dies-irae-sequentia.html) showing the complete text as Latin poetry plus a link to a Gregorian chant rendition.