Question: The fictional character in J. K. Rowling's "Harry Potter" series Severus Snape becomes a double-agent on behalf of this secret organisation to fight Voldemort and his followers known as what?

Context articles: Severus Snape
Harry Potter and the Deathly Hallows
Order of the Phoenix (fictional organisation)
Harry Potter and the Deathly Hallows – Part 2
Severus Snape and the Marauders
Lord Voldemort
Harry Potter and the Philosopher's Stone
John Nettleship
Harry Potter and the Half-Blood Prince
Ministry of Magic

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 Deathly Hallows is the seventh and final novel of the "Harry Potter" series, written by British author J. K. Rowling.
 The book was released on 21 July 2007, ten years after publication of "Harry Potter and the Philosopher's Stone" (1997), by Bloomsbury Publishing in the United Kingdom, in the United States by Scholastic, and in Canada by Raincoast Books, ending the series that began in 1997 with the publication of "Harry Potter and the Philosopher's Stone".
 The novel chronicles the events directly following "Harry Potter and the Half-Blood Prince" (2005), and the final confrontation between the wizards Harry Potter and Lord Voldemort, as well as revealing the previously concealed back story of several main characters.
 The title of the book refers to three mythical objects featured in the story, collectively known as the "Deathly Hallows"—an unbeatable wand, a stone to bring the dead to life, and a cloak of invisibility.
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".
Harry Potter and the Deathly Hallows – Part 2 is a 2011 British-American fantasy film directed by David Yates and distributed by Warner Bros.
 Pictures.
 It is the second of two cinematic parts based on the novel of the same name by J. K. Rowling.
 The film, which is the eighth and final instalment in the "Harry Potter" film series, was written by Steve Kloves and produced by David Heyman, David Barron, and Rowling.
 It is the sequel to "Harry Potter and the Deathly Hallows – Part 1".
 The story continues to follow Harry Potter's quest to find and destroy Lord Voldemort's Horcruxes in order to stop him once and for all.
Severus Snape and the Marauders is a 2016 American short film written by director Justin Zagri, based on "Harry Potter" characters by J. K. Rowling.
 It officially premiered on March 1, 2016 at the Youtube channel Broad Strokes Productions.
 The fan film caught the attention of BuzzFeed, "Entertainment Weekly", "Time",
Lord Voldemort ( , or in the films; born Tom Marvolo Riddle) is a fictional character and the main antagonist in J. K. Rowling's series of "Harry Potter" novels.
 Voldemort first appeared in "Harry Potter and the Philosopher's Stone", which was released in 1997.
 Voldemort appears either in person or in flashbacks in each book and its film adaptation in the series, except the third, "Harry Potter and the Prisoner of Azkaban", where he is only mentioned.
Harry Potter and the Philosopher's Stone is the first novel in the "Harry Potter" series and J. K. Rowling's debut novel, first published in 1997 by Bloomsbury.
 It was published in the United States as Harry Potter and the Sorcerer's Stone by Scholastic Corporation in 1998.
 The plot follows Harry Potter, a young wizard who discovers his magical heritage as he makes close friends and a few enemies in his first year at the Hogwarts School of Witchcraft and Wizardry.
 With the help of his friends, Harry faces an attempted comeback by the dark wizard Lord Voldemort, who killed Harry's parents, but failed to kill Harry when he was just 15 months old.
John Lawrence Nettleship (1 August 1939 – 12 March 2011) was a British schoolteacher who taught chemistry at Wyedean School, Gloucestershire.
 His pupils there included Joanne Rowling, whose mother worked for some time as an assistant in his department.
 He has been stated to be a major inspiration for the character of Severus Snape in Rowling's "Harry Potter" series of fantasy books.
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.
The Ministry of Magic is the government of the Magical community of Britain in J. K. Rowling's "Harry Potter" series.
 First mentioned in "Harry Potter and the Philosopher's Stone," the Ministry makes its first proper appearance in "Harry Potter and the Order of the Phoenix".
 Throughout the books, it is generally depicted as either corrupt, incompetent, or both, with its high officials blind to actual events and dangers.
 It reaches a nadir of corruption before being effectively taken over by Lord Voldemort.
 At the end of the final book, following Voldemort's death, Kingsley Shacklebolt takes over the ministry, changing it for the better.
 By the time of "Harry Potter and the Cursed Child", Hermione Granger is the Minister for Magic.