At last, the twilight deepens — soon it will be the darkness of evening.
Literal
Finally dusk, soon evening-darkness.
薄暮 (はくぼ) 'dusk / twilight' is a literary compound (薄 'thin/pale' + 暮 'evening'). 宵闇 (よいやみ) 'the darkness of early evening' is another literary compound (宵 'early evening' + 闇 'darkness'). いよいよ 'at last / finally' marks the transition. The sentence is pure atmospheric description with no verb — a poetic fragment.