The Syndicate is an extremley powerful international terrorist organization, which was a former proposed project to perform missions without an oversight, making the British Prime Minister an executioner without accountability. They were known for years as the main adversaries to Jim Phelps, Ethan Hunt and the Impossible Missions Force.
MI6 director Atlee has created it within the MI6 without any permission from the Prime Minister, and rejected by the Prime Minister himself, Atlee went rogue and turned it into a criminal organization
In 1999, The Syndicate conspired with Swanbeck, who was the IMF Mission Commander, to trick Ethan Hunt to deliver the Chimera Virus and then eliminate both of him and Nyah Nordoff-Hall. However, since Ethan destroyed the virus and Nyah´s criminal record was cleaned, The Syndicate had Swanbeck killed in Paris.
Taken over by Solomon Lane
In some point, Solomon Lane took up the project and turned to lead the Syndicate. It is consisted of numerous governmental agents who betrayed their homeland and faked their deaths, all of them went rogue, joined in the Syndicate with Lane as the leader while those operatives became assassins and agents for the Syndicate.
Rogue Nation Incident
On Feburary 2014, the Syndicate reveal their existence to Hunt, having infiltrated the station to subdue him. Hunt is rescued by a disavowed British agent Ilsa Faust, a Syndicate operative who offers no explanation for her actions. With their existence confirmed, Hunt begins following his only lead: a blond man in glasses who oversaw his capture, which is later revealed to be the organization's top leader; Solomon Lane.
Six months later, Faust partners with Hunt and Dunn to locate a file hidden by a Syndicate traitor on a high-security server used to regulate a Moroccan power station. Believing it to be a ledger containing the names of all Syndicate agents, the trio secure it before Ilsa betrays Hunt and Dunn. She returns to London, passing the file on to her handler,Attlee. However, Attlee discreetly erases the file, forcing Ilsa to return to the Syndicate and prove her loyalty to its leader, Solomon Lane, who Hunt knows as the man in the glasses.
Having betrayed one another so many times, they can no longer be certain of their own loyalty, question what to do next as they cannot be sure that Lane has not anticipated and moved to manipulate them. When Dunn and Ilsa are abducted by Lane's men, Hunt realizes that Lane will always have a plan to acquire the files, and that the only way to stop him is to force a confrontation with him. He agrees to Lane's ultimatum to abduct the British Prime Minister and use his voice print to unlock the file.
At a final stand, disturbed by this turn of events, Brandt contacts Hunley and reveals their location. Alan Hunley arrives at a charity auction to try and prevent Hunt from attacking the Prime Minister, and take him with Brandt and Atlee to a secure room. Having posed as Atlee, Hunt reveals himself and has the Prime Minister confirm the existence of the Syndicate: a top-secret project that sought to recruit all disavowed foreign governmental agents to carry out clandestine missions; an initiative that the Prime Minister rejected as too extreme. When the real Attlee arrives, Hunt subdues him, and he admits that Lane hijacked the project for his own ends and went rogue, and that Atlee has been covering up its existence ever since. Atlee is arrested and is taken into custody.
Later, Stickell reveals that the file is not a ledger of all agents, but contains the location and access codes for billions of dollars in untraceable funds that Lane has been desperate to get in order to keep it operational for decades. Hunt memorizes the data and destroys the file to force Lane to release Dunn and Ilsa in exchange for what he knows. He lures Lane out into the open and into a trap set up by Stickell. Imprisoned in a bulletproof cell, he is gassed and taken into custody for his crimes and the Syndicate is apparently dismantled.
Behind the scenes
The Syndicate are the main antagonists of the Mission: Impossible Cinematic Universe. Their role pays homage to The Syndicate from the original Mission: Impossible TV show, where they appeared as the key antagonistic organization of the series and the IMF's number one enemy as well.