Acherontic vs underworld

Acherontic

adj
  • Of, pertaining to or resembling Acheron (one of the rivers located in the underworld according to ancient Greek mythology). 

  • Lacking joy and comfort; nearing death. 

  • Of or pertaining to hell. 

underworld

noun
  • That part of society that is engaged in crime or vice. 

  • The portion of a game that is set below ground. 

  • The world of the dead, located underneath the world of the living; the afterlife. 

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