Kyrie IV (Altissime, Cantus ad libitum)

This is the beautiful Kyrie (Altissime) from Mass IV, sung by the Benedictine monks of St. Martin Beuron.

 

According to the Liber Usualis (1961), Mass IV (Cunctípotens génitor Deus) is "For Feasts of the II Class."   This is a Kyrie Cantus ad libitum, an option to be used, according to a note in the L.U., "in order to add greater solemnity."

Cunctípotens génitor Deus is "Omnipotent Creator God" in English.  This nine-fold Kyrie - in common with other nine-fold Kyries - has also been used in a medieval trope.   Here's a definition of "trope," from the Episcopal Church website; it includes something about Cunctípotens génitor Deus, too:

A textual insertion into the authorized liturgical texts. Tropes varied from a few words to lengthy sentences. Used with traditional plainchant, the extra words were matched to the notes of a long melisma (a series of notes assigned to one syllable of the text). For example, the setting for the Kyrie eleison at S 356 in The Hymnal 1982 Accompaniment Edition, Vol. 1, once included the trope cunctipotens Genitor Deus inserted after the word Kyrie. The practice of including tropes was popular in the ninth to thirteenth centuries.

This site has the words to - and quite a bit more about - Cunctípotens génitor Deus.  As you can see, the words to each of the nine stanzas can be fitted to the chant, in lieu of the usual Kyrie text:

Cunctipotens genitor Deus omnicreator eleison    
Salvificet pietas tua nos bone rector eleison    
Fons et origo bone pie luxque perhennis eleison    
Christe dei splendor virtus patrisque sophia eleison    
Plasmatis humanis factor lapsis reparator eleison    
Ne tua dampnatur Jhesu factura benigne eleison    
Amborum sacrum spiramen nexus amorque eleison    
Procedens fomes vite fons purificans vis eleison    
Indultor culpe venie largitor optime offensas dele sacro nos munere reple eleison    
Spirte alme eleison    

All-powerful Father, God, Creator of all things, have mercy
May thy compassion save us, good ruler, have mercy
Font and origin of goodness, Holy one, light everlasting, have mercy
Christ, the splendor of God, strength and wisdom of the Father, have mercy
Creator of humankind, healer of those who fall, have mercy
Lest thy creation be damned, kind Jesus, have mercy
The holy breath, the fusion and the love of both, have mercy
Advancing flame, source of life, purifying power, have mercy
Forgiver of sin, bestower of pardon, erase our offenses, replenish us, give us holy grace, have mercy
Most gracious Spirit, have mercy

A beautiful Trinitarian text!  That same site also offers this:

Generally Kyries of the early medieval period contained long untexted portions. To compensate for this, Notker writes that he fitted Latin poetry to the untexted portions as a mnemotic device. Consequently, the Kyrie began to exist in two equally valid formats: the original Greek text and in a Latin prosula. In this case, prosula refers to the words added to the pre-existing chant. Such additions, known also as tropes were added to many chants, in both the Mass and the Office, that contained long melismatic passages. In the Mass this showed itself most clearly in the Kyrie and the Alleluia. Adding words, however, could become problematic, as tradition held that the words and music of the chants were given to the original composers by God himself. These added texts, nevertheless, served three purposes: to provide a memory aid for long melismas; to enhance and elaborate the liturgy; and (in a backhanded way) to create new liturgical texts. Tropes and Sequences (another form of medieval liturgical poetry) were severly restricted in the reforms of the 16th century Council of Trent: four Sequences were allowed to remain and all tropes were eliminated.

It is believed that a man named Tuotilo was responsible for writing this added text to Kyrie IV. Like Notker, Tuotilo was a monk at the monastery of St. Gall. Also like Notker, Tuotilo was a student of Iso annd Marcellus, unlike Notker, Tuotilo was also a sculptor and painter. Scholars believe that Notker and Tuotilo shared the work of composing prosulas; Notker was primarily responsible for adding text to the Alleluias and Tuotilo added text to other parts of the Mass. Ekkehard IV, who wrote a small biography of these early monks described Tuotilo's melodies as "strange and easily recognisable."  Of Tuotilo himself, he wrote:

"Tutilo was widely different. [from Notker] He was strong and supple in arm and limb, such a man as Fabius tells us to choose for an athlete; ready of speech, clear of voice, a delicate carver and painter; musical, with especial skill on the harp and the flute; for the Abbot gave him a cell wherein he taught the harp to the sons of noble families around. He was a crafty messenger, to run far or near; skilled in building and all the kindred arts; he had a natural gift of ready and forcible expression whether in German or in Latin, in earnest or in jest; so that the emperor Charles [the Fat] once said, "Devil take the fellow who made so gifted a man into a monk!" But with all this he had higher gifts: in choir he was mighty, and in secret prayer he had the gift of tears; a most excellent composer of poetry and melodies, yet chaste, as became the disciple of our Master Marcellus, who shut his eyes against women."

Here's something from Wikipedia about tropes, with the text for another troped Kyrie:

Tropes were a particular feature of the music and texts of the Sarum Use (the use of Salisbury, the standard liturgical use of England until the Reformation), although they occurred widely in the Latin church. Deus creator omnium, was widely used in the Sarum Use and is in the form of a troped Kyrie.

Deus creator omnium tu theos ymon nostri pie eleyson.

Tibi laudes coniubilantes regum rex Christe oramus te eleyson.

Laus virtus pax et imperium cui est semper sine fine eleyson.

Christe rex unice Patris almi nate coeterne eleyson.

Qui perditum hominem salvasti de morte reddens vite eleyson.

Ne pereant pascue oves tue Jesu pastor bone eleyson.

Consolator Spiritus supplices ymas te exoramus eleyson.

Virtus nostra Domine atque salus nostra in eternum eleyson.

Summe Deus et une vite dona nobis tribue misertus nostrique tu digneris eleyson.

O God, creator of all things, most benevolent God: have mercy upon us.

To you, Christ, King of Kings, we pray and rejoice together: have mercy.

Praise, strength, peace and power are given to him always and without end: have mercy.

Christ, king coeternal and only-begotten of the father: have mercy.

Who saved lost man from death and restored him to life: have mercy.

Jesus, good shepherd, let not your sheep perish: have mercy.

Holy Spirit, the Comforter, we implore you to pray for us: have mercy.

Lord God our strength and salvation in eternity: have mercy.

Great and ever-living God, you have had pity on us. Grant your gifts to those whom you deem worthy: have mercy.

The standard Latin-rite ninefold Kyrie is the backbone of this trope. Although the supplicatory format ('eleyson'/'have mercy') has been retained, the Kyrie in this troped format adopts a distinctly Trinitarian cast with a tercet address to the Holy Spirit which is not present in the standard Kyrie. Deus creator omnium is thus a fine example of the literary and doctrinal sophistication of some of the tropes used in the Latin rite and its derived uses in the mediæval period.

The troped material for the kyrie in the video has been dropped out, leaving just the original form of the chant, Kyrie eleison, Christe eleison, Kyrie eleison.

Some of the masses are actually named according to the tropes on their Kyries, as far as I can tell.  (I am in the middle of some investigation into tropes, and will confirm this when I'm sure of it!)

There's more about tropes here, at New Advent.