وَمِنْ شَرِّ غَاسِقٍ إِذَا وَقَبَ
And from the harm of the dark night when it descends. (113:3)
Ghāsiq is indefinite and genitive as the complement of sharr. The familiar explanation is the darkening night. Idhā waqab directs attention to the time when that darkness enters or spreads. The prayer concerns evil associated with that circumstance, not a claim that night as such is intrinsically evil.
The temporal force of idhā matters; it should not be mechanically reduced to an ordinary future hypothetical without its context.