death knell vs death sentence

death knell

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

  • The tolling of a bell announcing death. 

death sentence

noun
  • Anything that spells death. 

  • A decree that someone be put to death as a punishment for a crime; a sentence of execution. 

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