Question: Who is Harry Potter and Albus Dumbledore's shared nemesis?

Context articles: Order of the Phoenix (fictional organisation)
Michael Gambon
Harry Potter (character)
Albus Dumbledore
Dumbledore's Army
Severus Snape
Harry Potter and the Chamber of Secrets (film)
Harry Potter and the Half-Blood Prince
Lego Harry Potter: Years 1–4
Richard Harris

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".
Sir Michael John Gambon {'1': ", '2': ", '3': ", '4': "} (born 19 October 1940) is an Irish-born English actor who has worked in theatre, television and film.
 Gambon has played the eponymous mystery writer protagonist in the BBC television serial "The Singing Detective", Jules Maigret in the 1990s ITV serial "Maigret", and Professor Albus Dumbledore in the final six "Harry Potter" films after the death of previous actor Richard Harris.
 He has won four BAFTA TV Awards and three Olivier Awards.
Harry James Potter is the title character and protagonist of J. K. Rowling's "Harry Potter" series.
 The majority of the books' plot covers seven years in the life of the orphan Potter, who, on his eleventh birthday, learns he is a wizard.
 Thus, he attends Hogwarts School of Witchcraft and Wizardry to practise magic under the guidance of the kindly headmaster Albus Dumbledore and other school professors.
 Harry also discovers that he is already famous throughout the novel's magical community, and that his fate is tied with that of Lord Voldemort, the internationally feared Dark Wizard and murderer of his parents, Lily and James.
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.
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".
Severus Snape is a fictional character in J. K. Rowling's "Harry Potter" series.
 An exceptionally skilful wizard, his coldly sarcastic and controlled exterior conceals deep emotions and anguish.
 A Professor at Hogwarts School of Witchcraft and Wizardry, Snape is hostile to the series' protagonist, Harry, the moment he sets foot in Snape's classroom; this hostility is rooted in Harry's resemblance to Harry's father James, who bullied and harassed Snape during their time at Hogwarts.
 Though philosophically attracted to the Dark Arts and Lord Voldemort's ideology of wizard supremacy, Snape's love for Muggle-born Lily Evans, Harry's mother, eventually compels him to defect from the Death Eaters.
 Snape becomes a double-agent on behalf of Albus Dumbledore and the Order of the Phoenix, and protects Harry throughout the series.
Harry Potter and the Chamber of Secrets is a 2002 British-American fantasy film directed by Chris Columbus and distributed by Warner Bros.
 Pictures.
 It is based on the novel of the same name by J. K. Rowling.
 The film is the second instalment in the long-running "Harry Potter" film series.
 It was written by Steve Kloves and produced by David Heyman.
 Its story follows Harry Potter's second year at Hogwarts School of Witchcraft and Wizardry as the Heir of Salazar Slytherin opens the Chamber of Secrets, unleashing a monster that petrifies the school's denizens.
 The film stars Daniel Radcliffe as Harry Potter, with Rupert Grint as Ron Weasley, and Emma Watson as Hermione Granger.
 The film is also the last film to feature Richard Harris as Professor Albus Dumbledore, due to his death that same year.
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.
Lego Harry Potter: Years 1–4 is a Lego-themed action-adventure video game developed by Traveller's Tales and published by Warner Bros.
 The game is based on the Lego Harry Potter line and its storyline covers the first four films in the "Harry Potter" series: "Harry Potter and the Sorcerer's Stone", "Harry Potter and the Chamber of Secrets", "Harry Potter and the Prisoner of Azkaban", and "Harry Potter and the Goblet of Fire".
 The game is available on the Wii, Xbox 360, PlayStation 3, Nintendo DS, PlayStation Portable, Microsoft Windows, OS X, iOS and Android.
 The OS X version of the game was released on 22 February 2011 by Feral Interactive.
 The game was released for the PlayStation 4 on October 21, 2016, as part of the "Lego Harry Potter Collection", which bundles the game with its sequel, "".
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".