For many years (1569-1584), the Earl and Countess had to keep Mary Queen of Scots imprisoned on one or another of their estates, but it was not until that poor woman was removed to another jailer that she got into the trouble that cost her her life. Around the same time she was removed from his custody, the Earl left Bess for good -- they had been separated off-and-on since about 1580, and even Queen Elizabeth had tried to get them to reconcile. (Queen Mary seems to have aggravated, if not created, their problems by playing them off against each other.) Bess's daughter Elizabeth Cavendish married Mary's brother-in-law Charles Stuart, and their only child Arbella Stuart had a claim to the thrones of Scotland and England.
Bess became famous for her building projects, especially two of them: Chatsworth, now the seat of the Dukes of Devonshire (whose family name is still "Cavendish," because they are descended from her children from her second marriage), and Hardwick Hall, of which it has been said for 400+ years now: "Hardwick Hall, more glass than wall," because of the number and size of its windows. She was interred in a vault in Derby Cathedral, where there is a memorial to her.
Bess Hardwick is the subject of Jan Westcott's novel The Tower and the Dream (1974).