cautionary tale vs omen

cautionary tale

noun
  • A story, originally told in folklore, to warn its audience of a certain danger. 

omen

noun
  • Something which portends or is perceived to portend either a good or evil event or circumstance in the future, or which causes a foreboding; a portent or augury. 

  • A thing of prophetic significance. 

verb
  • To divine or predict from omens. 

  • To be an omen of. 

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