Hadean vs tenebrous

Hadean

adj

  • Pertaining to Hades or hell. 

  • Of, or relating to the geological eon. 

name

  • The Hadean eon; part of the Precambrian supereon, spanning from around 4.6 to 4 billion years ago. 

tenebrous

adj

  • dark and gloomy 

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