sempiternally vs unrelentingly

sempiternally

adv
  • In a sempiternal manner; eternally. 

  • in an everlasting manner, as to have an infinite temporal duration; as opposed to eternally or timelessly, as to exist outside time 

unrelentingly

adv
  • In an unrelenting manner; rigidly; cruelly. 

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