The enigmatic Borg Queen is the central locus of the Borg Collective. She brings order to the legions of voices within the Hive mind and provides a common direction — much like the queen of an insect colony. She resides primarily at Unimatrix One in the Delta Quadrant, but will often leave this home base to participate in assimilation efforts of a special nature.
The Borg Queen has a unique personality and a sense of individuality that normal Borg drones are not allowed. She is usually the one who "speaks" for the Collective in situations where contact with outsiders is best conducted by an individual. But for the Borg Queen the concepts "I" and "we" are interchangeable. In her own words, she is the "one who is many."
The Queen spends much of her time in her "lair" with her head and spinal column residing in a special alcove. When she emerges, she will "re-assemble" herself into a predominantly artificial body — the arms, legs and torso appearing to be entirely synthetic, while the head and shoulders seeming to be organic, but with substantial cybernetic implants.
Information on this being is still very limited. Apparently the Borg Queen has been destroyed on a number of occasions, but another queen always seems to take her place. (It is not clear whether more than one queen exists simultaneously, or if a new queen is created when the old one dies.) The Borg's collective nature makes it likely that each Borg Queen has all her predecessors' (and/or counterparts') qualities and memories. Therefore when she speaks as "I," she is presumably referring to all previous manifestations of the Queen, going back probably thousands of years.
The earliest Federation data on the Borg Queen was collected by Magnus and Erin Hansen, two exo-biologists who spent several years studying the Borg in the 2350's aboard their science vessel, the U.S.S. Raven. The Hansens were assimilated before they could transmit their findings to Starfleet, but the information was later recovered by the U.S.S. Voyager about 20 years later.
In 2373, the Borg Queen spearheaded the Collective's attempt to alter Earth's history in order to make humanity easier to assimilate. However, due to interdiction by the U.S.S. Enterprise-E, not only did the plan fail, but the Queen and all other Borg involved — who were occupying the Enterprise at the time — were killed when plasma coolant liquefied their organic components.
This Queen was soon replaced by another, almost identical one — a drone assimilated from Species 125 — who encountered the U.S.S. Voyager in the Delta Quadrant when that Federation crew attempted to procure a transwarp coil. The Borg Queen Vessel was destroyed during that incident; however, it is not clear if the Queen herself was still aboard at the time. If so, she was replaced by another virtually-identical successor, whom Voyager once again encountered, in 2377, at a time when the so-called "Unimatrix Zero" phenomenon threatened the integrity of the Collective. That Queen apparently died in 2378, when a neurolytic pathogen was introduced into the Hive and wreaked havoc on a massive scale.
At this time, it is not known if the Queen was subsequently replicated, or even if the Borg Collective survived the pathogenic attack.
The assimilation of the human race has been an elusive goal of the Borg since its first encounter with the U.S.S. Enterprise-D in 2365, in spite of that species' below-average cranial capacity and other physiological limitations. The Borg Queen has long sought a counterpart with a mind of his or her own to help bridge the gulf between humanity and the Borg — someone to be more than just another drone. In late 2366, during the Borg incursion that culminated in the costly battle at Wolf 359, the Queen wanted Captain Jean-Luc Picard to give himself freely to the Borg, but he resisted and was transformed into Locutus. (He was soon separated from the Collective, however.) Seven years later, during the Borg's temporal incursion to Earth in the year 2063, the Queen tried to similarly seduce the android Data, by giving him organic components and by appealing to his emotions, but again, resistance proved to be not futile.
Her successor later "allowed" the drone Seven of Nine (who had been the daughter of the Hansens — probably the first humans ever assimilated) to be separated from the Collective by the U.S.S. Voyager, in order for her to gain experience as an individual. Almost two years later, in 2375, the Queen attempted to lure Seven back into the Collective — again, by having her give herself voluntarily to the goal of Borg "perfection", while remaining an individual within the Hive. After refamiliarizing Seven with assimilation procedures during an attack upon Species 10026, the Queen ordered her to program nanoprobe viruses that would be used surreptitiously upon Species 5618 — the human race.
In spite of the Queen's seductive appeals to the former drone, Seven of Nine refused to assist in the destruction of yet another race, especially the one she had come from and was now living among. The Queen even brought in the Magnus Hansen drone to convince Seven that her "family" was the Collective. These temptations ultimately failed, and Seven was rescued by Captain Janeway and the U.S.S. Voyager, to the Queen's chagrin.
In 2376-2377, the Queen was engaged in an obsessive quest to uncover the interlink frequency that bound certain drones in a dreamstate realm called Unimatrix Zero while regenerating. She wished to terminate this realm, because in it drones were able to regain their identities as individuals, and even though they would awaken from it without any memory of the experience, the Queen considered it a threat to her control over the Hive. Thanks to intervention by Voyager, the Queen actually got her wish, but not quite how she wanted: Unimatrix Zero was shut down, but the drones who occupied it regained their individuality in the physical world and began a resistance movement to undermine the Collective.
In 2378 the Queen once again had a fateful encounter with Voyager when a future version of Kathryn Janeway came back in time from the year 2404 with technology that could help the lost starship resist the Borg while using a Transwarp Hub as a shortcut home. The Queen was actually approached by Admiral Janeway with an "offer" — to help the Queen prevent Voyager from destroying the Transwarp Hub in exchange for the ship's safe passage to the Alpha Quadrant. But the Queen wanted more — she wanted to assimilate the technology from the future. The Queen thought she had the upper hand when she attempted to assimilate the admiral, but it turns out the older Janeway was carrying a neurolytic pathogen that proceeded to infect the Collective. That pathogen brought chaos to the Collective, leading to the decapacitation of the Queen and the destruction of Unimatrix One.
The extent of the damage to the Borg as a whole, and its status as a continued threat to the galaxy, remains unknown at this time.