This prayer comes from Anglican Use Evensong, and had originally been the concluding prayer of the Sarum Compline. Here is the Sarum original:
Illumina, quaesumus, Domine, tenebras nostras: et totius huius noctis insidias tu a nobis repelle propitius.
In other words, Cranmer, as his custom was, expanded propitius to by thy great mercy and insidias ['ambushes'] to perils and dangers. the ambushes of this whole night thus became all perils and dangers of this night.
Just as Cranmer padded and expanded, lest his vernacular version of the prayer be finished before the worshippers had quite tuned in to it, so, through the Middle Ages, this prayer had already grown in the Latin. Here is the version in the 'Gregorian Sacramentary', with those words crossed out which were subsequently added.
Illumina, quaesumus, Domine, tenebras nostras: et totius
But what will really surprise you is the Heading a little way above it the 'Gregorian Sacramentary'.
INCIPIUNT ORATIONES MATUTINALES
Gracious! It was apparently a collect for the Dawn!! It did not ask for God to protect us through the darkness of this night; it asked God to push away (repelle) the dangerous darkness of night. Look back at Latin text!
[It may be that I am wrong. Another prayer in this section does look like a late evening prayer, so perhaps the Heading is erroneous. Illumina is certainly an evening prayer in the 'Gelasian Sacramentary'. But this exercise may serve to remind us how things are not always what they seem!]