existence vs nightmare

existence

noun
  • Empirical reality; the substance of the physical universe. (Dictionary of Philosophy; 1968) 

  • The state of being, existing, or occurring; beinghood. 

nightmare

verb
  • To have a nightmare. 

noun
  • Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure. 

  • A very bad or frightening dream. 

  • A feeling of extreme anxiety or suffocation experienced during sleep; Sleep paralysis. 

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