This is the 8727th most frequent Japanese word.
暗し
"暗し" (kurashi) means "dark" or "gloomy" in an archaic or literary sense.
The word 暗し is used as an adjective here to describe the characteristic of the room being dark.
部屋が暗し、灯をつけましょう。
The room is dark, let's turn on the light.
In this sentence, 暗し serves to describe a situation of metaphorical darkness, indicating difficulty or sadness.
暗しながらも心は晴れやかだ。
Though it's dark, the heart remains cheerful.
Here, 暗し describes the physical state of the forest being dark, emphasizing the unusual nature of needing light during daytime.