death knell vs portent

death knell

noun
  • A sign or omen foretelling the death or destruction of something. 

  • The tolling of a bell announcing death. 

portent

noun
  • Something that portends an event about to occur, especially an unfortunate or evil event; an omen. 

  • A portending; significance 

  • Something regarded as portentous; a marvel; prodigy. 

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