Bloody Mary was a sobriquet given to Mary I (1516-1558), the first queen regnant of England from 1553 to 1558. The daughter of Henry VIII and his first wife Catherine of Aragon, Mary inherited her mother's (and her father's, early in his life) staunch Catholicism.

Her predecessor was her younger brother, Edward VI (strongly protestant), who died at the age of 15. Fearing that she would reverse his reforms and their father's, plans were made to exclude her from the line of succession, which incidentally also excluded her sister, the future Elizabeth I. Under those terms, the crown next passed to the 17-year old Jane Gray, who was deposed after less than two weeks and executed and is now not counted in the list of British monarchs.

Soon after her accession, Mary (who had married the King of Spain Philip II, also staunchly Catholic) had several leading Protestant churchmen imprisoned. In 1553, all of the religious legislation promoted under her brother's reign, mainly by Thomas Cranmer, Archbishop of Canterbury, were repealed, reaffirming, among others, clerical celibacy.

In 1554, the Heresy Acts, which had been repealed under Henry VIII and Edward VI, were revived. Under those, numerous Protestants were executed, under what is now known as the Marian Persecutions, with several bishops being burned at the stake; 283 people were executed as a whole, which even some of Philip II's ecclesiastical staff condemned.

As a whole, she played absolutely no role in the Lutheran reformation itself, being all of one year old when the 95 Thesis were posted. She was viewed as a bloodthirsty tyrant soon after her reign, with England swiftly turning protestant under her sister Elizabeth and their successors. Her sobriquet was known to pop up as early as the 17th century. Historical revisionism has improved her reputation some (her death toll being small when compared to her father's), but she is still a hotly debated subject among historians today.

Hope this helped!