It wasn't until nightfall that we finally reached the foot of the mountain.
Literal
Night [to-に]-becoming-[て] finally mountain [gen-の] foot [to-へ] arrived.
~になって ('having become X,' te-form of なる) marks the temporal transition. やっと ('finally, at last') emphasizes the long-awaited arrival. 山のふもと ('the foot of the mountain') uses ふもと ('foot, base') for mountain bases. たどり着く ('arrive after a hard journey') is a compound verb specifically for reaching a destination after difficulty.