Professor Moriarty

Professor James Moriarty is a fictional character in some of the Sherlock Holmes stories written by Sir Arthur Conan Doyle. Moriarty is a criminal mastermind whom Holmes describes as the "Napoleon of crime". 
Doyle got the phrase from a Scotland Yard inspector. The inspector was referring to a real-life criminal. The character was introduced to allow Doyle to kill Sherlock Holmes, and only featured in two of the Sherlock Holmes stories. Later, he has often been treated as Holmes' archenemy.
Professor Moriarty's first and only appearance occurred in "The Adventure of the Final Problem". There Holmes is about to deliver a fatal blow to Moriarty's criminal organization. Suddenly he is forced to flee to continental Europe to escape Moriarty's revenge. Moriarty follows. 
The pursuit ends in Switzerland on top of the Reichenbach Falls. The encounter that apparently ends with both Holmes and Moriarty falling to their deaths.
Moriarty also plays a role in the book-length "The Valley of Fear", and his agents are mentioned in a few other stories.