cloak and dagger vs mystery

cloak and dagger

mystery

noun
  • Someone or something with an obscure or puzzling nature. 

  • Something secret or unexplainable; an unknown. 

  • A religious truth not understandable by the application of human reason alone (without divine aid). 

  • A particular event or series of events in the life of Christ. 

  • A sacrament. 

  • A secret religious celebration, admission to which was usually through initiation. 

  • A craft, art or trade; specifically a guild of craftsmen. 

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