Page "History of Doctor Who" Paragraph 27

Another innovation of theirs from the eighth season onwards was the introduction of the character of the Master as a new nemesis for the Doctor, conceived as a Professor Moriarty to the Doctor's Sherlock Holmes.

Played by Roger Delgado, he became a highly popular character, although over the following two seasons it was felt that he became a little overused.

Delgado and the production team eventually agreed that he should be written out during the eleventh season by killing the character off, with some ambiguity as to whether or not he had died to save the Doctor.