descend upon vs flatten out

descend upon

verb
  • to visit in large numbers 

flatten out

verb
  • To win a fight, usually by punching someone until they are knocked over 

  • To make flat (something rough, folded or bumpy). 

  • To become more even. 

How often have the words descend upon and flatten out occurred in a corpus of books? (source: Google Ngram Viewer )