Question: Severus Snape becomes a double-agent in the secret organization founded by whom?

Context articles: Sarasota Assassination Society
Severus Snape
Potions in Harry Potter
Seven Wonders (series)
Order of the Phoenix (fictional organisation)
Korean Patriotic Organization
Severus Snape and the Marauders
Abram D. Smith
John Nettleship
Alan Rickman

The Sarasota Assassination Society, also known as the Sarasota Vigilance Committee, was a late 19th-century secret organization established by Alfred Bidwell, Dr. Leonard Andrews and Jason Alford in Manatee County, Florida.
 The organization, which was estimated to include twenty to twenty-two members, was purported to be a political social group or a Democratic Club.
 The true aims of the group were chronicled in the February 2, 1885 "New York Times" article; “This organization is supposed to exist for the purpose of the secret murder of political opponents, and is composed of 20 members, bound together by terrible oaths to perform the bloody work of the band and to keep its secrets inviolate.”
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.
This is a list of potions used in the "Harry Potter" series.
 Many are mentioned in potions class at Hogwarts, which is taught by Severus Snape in books one to five and Horace Slughorn in books six and seven.
Seven Wonders is a pentalogy of children's fantasy, adventure and mythological fiction books written by American author Peter Lerangis.
 It is based on Greek mythology and set around the Seven Wonders of the Ancient World.
 Jack McKinley discovers a secret organization on a hidden island, and becomes the leader of a mission to retrieve seven lost magical orbs.
 As Jack and his three friends realize their lives are at stake, they have no choice but to accept the quest and embark on the challenge.
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".
Korean Patriotic Organization (), also known as Korean Patriotic Corps or Korean Patriotic Legion, was a secret organization that aimed to assassinate prominent Japanese figures of the Empire of Japan.
 It was one of the independence movements propelled by the Provisional Government of the Republic of Korea.
 The Korean Patriotic Organization was founded by Kim Gu, president of the Provisional Government of the Republic of Korea.
 Its executives were Kim Suk, Ahn Gong-geun, Lee Su-bong, and Lee Yu-pil.
 Notable members included Yoo Sang-geun, Yoo Jin-man, Yun Bong-gil, Lee Bong-chang, Lee Duk-ju, and Choi Heung-sik.
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",
Abram Daniel Smith (June 9, 1811 – June 3, 1865) was a leader of the Hunters' Lodges, a secret organization founded in 1838 in support of the Canadian Rebellions of 1837.
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.
Alan Sidney Patrick Rickman (21 February 1946 – 14 January 2016) was an English actor and director known for playing a variety of roles on stage and on screen.
 Rickman trained at the Royal Academy of Dramatic Art in London, and was a member of the Royal Shakespeare Company, performing in modern and classical theatre productions.
 His first big television part came in 1982, but his big break was as the Vicomte de Valmont in the stage production of "Les Liaisons Dangereuses" in 1985, for which he was nominated for a Tony Award.
 Rickman gained wider notice for his film performances as Hans Gruber in "Die Hard" and Severus Snape in the "Harry Potter" film series.