question: Who is Harry Potter and Albus Dumbledore's shared nemesis?
answer: Lord Voldemort
context: Title: Order of the Phoenix (fictional organisation)
  Passage: The Order of the Phoenix is a secret organisation in the "Harry Potter" series of fiction books written by J. K. Rowling.  Founded by Albus Dumbledore to fight Lord Voldemort and his followers, the Death Eaters, the Order lends its name to the fifth book of the series, "Harry Potter and the Order of the Phoenix".
  Title: Richard Harris
  Passage: Richard St John Harris (1 October 1930 – 25 October 2002) was an Irish actor and singer.  He appeared on stage and in many films, appearing as Frank Machin in "This Sporting Life", for which he was nominated for the Academy Award for Best Actor, King Arthur in the 1967 film "Camelot" and the subsequent 1981 revival of the show.  He played an aristocrat and prisoner in "A Man Called Horse" (1970), a gunfighter in Clint Eastwood's Western film "Unforgiven" (1992), Emperor Marcus Aurelius in "Gladiator" (2000), and Albus Dumbledore in the first two "Harry Potter" films: "the Philosopher's Stone" (2001) and "the Chamber of Secrets" (2002).  Harris had a number one hit in Australia and Canada and a top ten hit in the United Kingdom and United States with his 1968 recording of Jimmy Webb's song "MacArthur Park".
  Title: Dumbledore's Army
  Passage: Dumbledore's Army (or D.A. for short) is a fictional student organisation in J. K. Rowling's "Harry Potter" series that is founded by the main characters, Harry Potter, Ron Weasley and Hermione Granger, to stand up against the regime of Hogwarts High Inquisitor Dolores Umbridge, as well as to learn practical Defence Against the Dark Arts.  It was founded in the fifth book, "Harry Potter and the Order of the Phoenix".  Cho Chang conceives of the abbreviation "D.A." and Ginny Weasley conceives of the name "Dumbledore's Army".
  Title: Harry Potter and the Half-Blood Prince
  Passage: Harry Potter and the Half-Blood Prince is the sixth and penultimate novel in the "Harry Potter" series, written by British author J. K. Rowling.  Set during protagonist Harry Potter's sixth year at Hogwarts, the novel explores the past of Harry's nemesis, Lord Voldemort, and Harry's preparations for the final battle against Voldemort alongside his headmaster and mentor Albus Dumbledore.
  Title: Albus Dumbledore
  Passage: Professor Albus Percival Wulfric Brian Dumbledore is a fictional character in J. K. Rowling's "Harry Potter" series.  For most of the series, he is the headmaster of the wizarding school Hogwarts.  As part of his backstory, it is revealed that he is the founder and leader of the Order of the Phoenix, an organisation dedicated to fighting Lord Voldemort.
citations: Harry Potter and the Half-Blood Prince Albus Dumbledore