megadeath vs night of the long knives

megadeath

noun
  • One million deaths, especially as a unit of measure in reference to nuclear warfare. 

night of the long knives

noun
  • A purge, in which opponents of a regime or political party are killed or removed. 

How often have the words megadeath and night of the long knives occurred in a corpus of books? (source: Google Ngram Viewer )