darkness
曚 centers on darkness, specifically the dim light of early dawn or dusk when the sun is obscured.
曚 combines 日 (sun) with 蒙, which likely contributes both sound and the idea of covering or dimness, suggesting the sun obscured by haze or twilight.
The sun 日 is covered by the dense, hazy shape 蒙, like a morning fog that dims the light. 曚 is that dusky darkness.
For モウ, imagine the sun moaning (モウ) as it struggles to shine through the thick morning haze, its light barely visible.