The Millennium, also sometimes erroneously known as the Dark Millennium, was a Sith sect founded by Darth Millennial. Its goal was to steer the galaxy into the right direction and manipulate destiny to provide the ideal circumstances for the birth and life of the Sith'ari, the Sith Messiah that would bring about the ultimate victory of the dark side.
The Millennium was founded by Darth Millennial on Dromund Kaas at the same time as the Dark Force religion. It was embedded within the church's clergy, the Prophets of the Dark Side, and only his most devout followers were admitted into the order. Over time it established a formidable network of intelligence assets through the setting up of local cults among the wealthy elite of other worlds, which was kept a secret from the ordinary Prophets.
At some point, a conflict erupted between the Millennium and the Prophets, who had grown jealous of the order's growing power. The Sith disciples warred among them themselves for some time before the Milennium gave up and left the planet. They subsequently established their major headquarters and temple on Kuat and maintained chapter houses on key worlds such as Eriadu, Corellia, Commenor, Fondor and Etti IV. They then bided their time, subtly changing the course of the galaxy to further their goals.
In the years following the Empire's creation, a Second Schism occurred between factions interpreting the prophecies differently. One group argued that the terrorist actions of the madman Voidshadow were not according to prophecy, while another, this one vastly smaller, believed Voidshadow to actually be the Sith'ari. For a time, he ruled this dissident order, but after his defeat by Imperial Investigator Cedric Trax, they rejoined the true Millennium under Darth Tilentis' leadership. This drew the attention of Trax, the Imperial Investigators, and eventually Darth Vader on the order, and for some years, they became mortal enemies. Trax eventually gave up, believing he had defeated the order, and although Vader knew better, his silence had been bought.
For the following years, the Millennium's main interest was the study of the Yuuzhan Vong, who were secretly infiltrating the galaxy, as well as keeping their appearance from becoming public knowledge. The Vong were apparent proof that the Force could be defeated, since it had been so in them, and the order struggled, mostly unsuccessfully, to replicate their biotechnology.
In more recent times, they foiled an assassination attempt on the Emperor by tipping him off, and secretly assisted Luke Skywalker in escaping a plot by the Emperor to have Emperor's Hand Mara Jade kill him in 4 ABY, since they knew that the time was not right for them to die.
In the Yuuzhan Vong War, the Millennium saw fit to act more directly, covertly aiding the Vong in infiltration and sabotage. By bringing about the chaos and pain of total war, they would accumulate enough dark side power for the Chosen of the Sith'ari, the person whose body was destined to be inherited by his spirit, to reach his or her full potential and assume the mantle.
According to Millennial's prophecies, the birth of the Sith'ari would be dictated by the events that lead up to the seeming destruction of the Sith. A series of portents would predict what would happen; among these events, the Clone wars, the rise of the Empire, the Galactic Civil War, and the Yuuzhan Vong Invasion were foretold, as was the eventual demise of Darth Bane' schismatic Order of the Sith Lords. To Millennial, Bane and the much later Palpatine were false prophets that would lead the Sith astray, leaving only his hardened core of true believers to implement the millennia-old destiny of the Sith.
The chief responsibility of the Millennium was to ensure that Galactic history progressed as had been foretold. In Millennial's mind, the Sith were only a small part of an eternal war between the Light and Dark Side of the Force, a war that had been fought in cycles since long before the formation of the Sith themselves. The Sith'ari was, to him and his followers, the ultimate being that would finally end the war and destroy the Light Side forever. But unless all requirements of the prophecy revealed to him were met, the war would continue, spanning eons more until the prophesied circumstances arose.
The Sith'ari would be more than man but somewhat less than god, the ultimate triumph of the Dark Side. A being of "luminous darkness", he would lead the Millennium to their final victory over the Light. What they envisioned was a society devoid of all emotion except devotion to the cause. The Sith'ari would plunge the galaxy into a state of total tyranny, were minds and, in time, individuals would cease to exist. All life would exist only to feed the Sith'ari's power, and he would rule gloriously over an eternity of pain, fanaticism and confusion. After a thousand years of such existence, gorging himself on the suffering of the galaxy's untold trillions, he would rise to challenge and defeat the Force itself, thereby ending all life everywhere and forever, leaving only the darkness of the void as a monument to his glory, and transcend to a state no mortal could possibly fathom.