grail vs raison d'être

grail

noun
  • Something eagerly sought or quested for. 

  • Small particles of earth; gravel. 

  • One of the small feathers of a hawk. 

  • The Holy Grail. 

  • A book of offices in the Roman Catholic Church; a gradual. 

raison d'être

noun
  • Reason for being. The claimed reason for the existence of something or someone; the sole or ultimate purpose of something or someone. 

How often have the words grail and raison d'être occurred in a corpus of books? (source: Google Ngram Viewer )