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A maths professor with a criminal mind, the only one on Holmes's level. In the first film he is just a silhouette pulling strings. In the second he becomes the central villain, profiting from war. Jared Harris plays the character, created by Arthur Conan Doyle.
Moriarty went into the first script just to set up sequels. In the 2009 film he appears little, almost always faceless, and is revealed as the owner of a bigger scheme: Blackwood and Adler were a distraction while he stole a part from a machine.
In the second film, James Moriarty buys shares in arms companies and wants to start a world war to get rich. He uses bombs to set France against Germany and kills Irene Adler to intimidate Holmes. He also threatens Watson and his wife.
Captured, Holmes is tortured, but he has already swapped the professor's personal notebook. The final talk is a game of chess between two geniuses. Moriarty still smiles and says war will come anyway, and the fight ends at the Reichenbach drop.
In the books, Moriarty appears only in "The Final Problem" (1893) and is mentioned in The Valley of Fear (1915). He is the "Napoleon of crime". The films give him far more room.
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