Attributed to Pope St. Gregory the Great (540-604). In the Roman Breviary this hymn is used at Vespers during Lent for both Sundays and the ferial Office from the first Sunday in Lent until the Friday before Passion Sunday. In the Liturgia Horarum it is used at Vespers for the Sunday Office from the first Sunday until the Saturday before Holy Week.
This movement probably has the most colorful registration of the set so far - Gambe vs Bourdon, followed by Cromorne vs 16 and 8 Bourdons, the first registration returns, but now with nazard and tierce alone in the pedale (bird calls?) finishing with flute 4' vs voix celeste and 16' in the pedale. Sheer magic!