The Adventure of the Final Problem

"The Adventure of the Final Problem," first published in 1893 and included in "The Memoirs of Sherlock Holmes," introduces the legendary criminal mastermind Professor Moriarty, who becomes Holmes' greatest opponent. Recognized by Conan Doyle as one of his top Holmes stories, it was initially meant to be the final installment in the series, but its climactic ending prompted a significant public reaction that led Doyle to revisit the character.