。
The veil of night is descending.
Literal
Night [gen-の] veil [subj-が] is-descending-[in-progress-つつある].
Highly literary. とばり ('veil, curtain, shroud') is a poetic metaphor; 夜のとばり ('the veil of night') is a fixed literary image for nightfall. ~つつある ('be in the process of') is a formal/literary progressive form, more elevated than ~ている. The whole sentence is stylized as prose poetry.