The Millennium Group was a centuries-old secret society that in modern times doubled as a criminal investigative consulting firm that offered consulting services to law enforcement. The organization was formed at the dawn of Christianity and was based upon Judeo-Christian endtime prophecies. (TXF: "Millennium")
The members of the Group believed that an apocalyptic event was inevitable and it was their job to guide the world through the coming crisis. (MM: "Owls") However, not all members shared the same beliefs. The Millennium Group was divided into at least two factions: the Roosters, who saw Armageddon in the specifically religious terms; and the Owls, who had a secular view concerning the end of the world. (MM: "Owls", "Roosters") In addition to the divide over the nature of this coming event, the factions were also divided over the timetable. The Roosters maintained that the event would take place on or around the year 2000, while the Owls believed that the event was decades away, and would correspond to a celestial event that was set to take place around the year 2060.
J. Edgar Hoover was the originator of the modern group and recruited several members from the Federal Bureau of Investigation. (MM: "Matryoshka") By 1996, the Group was composed mostly of former FBI agents. (MM: "Owls", "Pilot")
The group was involved in a number of activities. Their primary role as far as the public was concerned was that of a law enforcement consulting agency, in which former law enforcement professionals and others with specialized knowledge and skills would offer their services to police departments on difficult or unusual cases. It was under the guise of a law enforcement agency that the group approached Frank Black and attempted to recruit him.
While Frank worked for the group as a consultant, he was never officially made a member, and over time he became suspicious of their true nature and motives.
Behind the facade of a law enforcement consulting agency, the group operated in extreme secrecy and behaved much like a cult, monitoring its own members, killing individuals whom the group considered threats, and even going so far as to execute its own members for various reasons. (MM: "Owls","Roosters", "The Fourth Horseman", "The Time is Now".)
The group had a number of unusual initiation rituals that seemed to come in stages. When a member had worked on the consulting side of the group for a certain amount of time and was considered "ready", they would be sent to meet with the Old Man, a mysterious elderly individual who lived in the mountains of Washington and was presumably the leader of the group. (MM: "Beware of the Dog".)
At some point after this, when the candidate was ready for initiation, they would take part in a ritual near a small river in the mountains, which involved cutting the hand with a ceremonial dagger. (MM:"The Fourth Horseman", "The Time is Now".)
The group had a long and somewhat unclear connection to the artificially developed Marburg virus, which Peter Watts claimed was developed by the Soviets and was connected to the Ebola virus, though the veracity of this is unclear. During the first gulf war, the group tested the virus on both Iraqi's and U.S. troops, and this may have had some connection to the gulf war syndrome, but the evidence was inconclusive. There is also a possibility that Peter Watts was personally involved in this operation. (MM: "Collateral Damage".)
The group would stage gruesome executions of various individuals, which would serve as a warning to group members arriving at a crime scene where one of these executions was found to behave in an appropriate manner, which presumably meant not revealing sensitive information to law enforcement.
While the group maintained that their goal was to guide mankind through the 'coming crisis', whatever form it may take, Frank Black came to believe that their true goal was not to prevent the events or guide man through them, but to use them as a method of control.
The group had extensive contacts with various levels of power, both public and private, and maintained an apparently extensive information gathering network. As of the late 1990s, the group was able to monitor and document the sale of certain types of books, as well as who purchased them. ("MM": 19:19)
The Millennium Group dissolved in 1999. Due to their involvement with the FBI, matters concerning the group were considered "sensitive". For that reason, Assistant Director Walter Skinner asked special agents Fox Mulder and Dana Scully to investigate a case that may have involved the remnants of the group. (TXF: "Millennium")