magnum opus vs masterstroke

magnum opus

noun
  • A great or important work of literature, music or art, a masterpiece. 

  • The best, most popular, or most renowned achievement of an author or artist, representing their major life effort. 

  • The process of working with the prima materia to create the philosopher's stone. 

masterstroke

noun
  • An action which demonstrates great skill or artistry. 

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