will o' the wisp vs wraith

will o' the wisp

noun
  • A flickering light or lantern, especially over bogs, swamps or marshes, held by a supernatural creature who uses it to lure travelers at night to their demise. 

  • A delusionary or otherwise unachievable goal that one feels compelled to pursue. 

wraith

noun
  • A ghost or specter, especially a person's likeness seen just after their death. 

How often have the words will o' the wisp and wraith occurred in a corpus of books? (source: Google Ngram Viewer )