。
Dusk gradually deepened.
Literal
Dusk [subject-が] gradually deepen-[go-away-ていった].
夕闇 ('dusk, twilight darkness,' literally 'evening darkness') is another poetic time-of-day word. 深まる ('to deepen,' intransitive) paired with ~ていく ('going on, progressing over time') — the opposite of ~てくる — gives the sense of the deepening progressing away from the speaker, i.e., into the future or continuing onward. 次第に ('gradually, little by little') is written/literary. Every word choice pulls this toward evocative prose.