adumbral vs adusk

adumbral

adj

  • Dark, shady. 

adusk

adj

  • Resembling conditions at dusk or twilight, low-lit, dark; in darkness, gloomy. 

adv

  • At dusk; in the dusk or twilight. 

How often have the words adumbral and adusk occurred in a corpus of books? (source: Google Ngram Viewer )