Moriarty is first introduced in the story "The Adventure of the Final Problem," where he is revealed to be the criminal mastermind behind a complex web of crimes that Holmes has been investigating. Moriarty is described as a "Napoleon of crime," and his criminal organization spans across Europe. He uses his intelligence and criminal network to commit various crimes, including murder, fraud, and blackmail.
Holmes confronts Moriarty in a dramatic showdown at the Reichenbach Falls, where they both seemingly fall to their deaths. However, it is later revealed that Holmes survived, and Moriarty's body is never found, leaving the possibility of his return open.
In subsequent Sherlock Holmes stories, Moriarty is mentioned or referenced as the epitome of evil and criminal genius. He is considered Holmes's greatest adversary, and his schemes and plans pose significant challenges to Holmes's detective skills.