relent vs slacken off

relent

verb
  • To slacken; to abate. 

  • To give in or be swayed; to become less hard, harsh, or cruel; to show clemency. 

noun
  • Stay; stop; delay. 

slacken off

verb
  • To slacken or lessen; to become less intense. 

  • To do (something) less intensely. 

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