internecine vs unrelenting

internecine

adj
  • Mutually destructive; most often applied to warfare. 

  • Characterized by struggle within a group, usually applied to an ethnic or familial relationship. 

unrelenting

adj
  • Not relenting; not yielding in strength, severity, or determination. 

  • Having no pity; not being or becoming lenient, mild, gentle, or merciful 

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