constantly vs unrelentingly

constantly

adv
  • In an unchangeable or invariable manner; in every case. 

  • In a constant manner; occurring continuously; persistently. 

  • Recurring regularly. 

unrelentingly

adv
  • In an unrelenting manner; rigidly; cruelly. 

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