In the mirror universe, the Borg were a race of cybernetically enhanced lifeforms which originated in the Delta Quadrant and spread, as the Borg Collective, throughout the galaxy, aiming to create a perfect race and bring order to the chaos of the galactic population by assimilating them—transforming them into drones of the Collective.
While the Borg in the mirror universe had the same hive-like mentality, the same assimilation methods, and the same advanced technology as their prime universe counterparts, the similarities ended there. The Borg of the mirror universe were proactively aggressive and attacked any intruder foolish enough to set foot on a Borg ship. Regardless of whether or not the intruder posed a threat to the ship, the Borg would use every resource at their disposal to assimilate the trespasser. In addition, while the Borg of the prime universe moved in a jerky, stiff-legged fashion, those of the mirror universe were much quicker thanks to advances made in nanoprobe technology. Attempts to aim a weapon at a mirror Borg, or avoid the injection tubules, were usually efforts in futility. In the mirror universe, the Borg are ruled over by a Borg King instead of a Borg Queen. (TNG - Mirror Universe novella: The Worst of Both Worlds)
Borg ships also differed in the mirror universe, as they were much larger. The cubes measured 10 kilometers or more on each side, and they were equipped with a vast array of weapons that could easily destroy a Vor'cha-class cruiser with a single shot. (Decipher RPG module: Through a Glass, Darkly)
Borg Invasion of 2371Edit
By 2371, the Borg had destroyed a series of Romulan and Klingon-Cardassian Alliance outposts along the Romulan Neutral Zone. These attacks attracted the attention of operatives of the Terran Rebellion, as well as Noonien Soong, a Terran scientist under Alliance employment. Believing the Borg could prove useful allies in their fight against the Alliance, Jean-Luc Picard and Soong set out to the neutral zone in an attempt to track down and contact the Borg. The pair were eventually able to locate the Borg in the Carraya system where they found a large diamond-shaped ship attacking an Alliance listening outpost on Carraya IV.
After assimilating the outpost, Soong (who became their spokesman Locutus of Borg) and a Klingon vessel, the Mek'leth, the diamond ship set course for Qo'noS, leaving behind a series of destroyed ships and colonies. Among the worlds devastated by the Borg were Beta Thoridor, Khitomer and Morska. In its wake were also a large number of refugee ships attempting to escape the slaughter and the impending fall of the Alliance.
It was in the Kowletz system, near Rura Penthe, in which a fleet of Klingon, Cardassian, Bajoran and Ferengi ships made a stand against the Borg; however, it was only through the efforts of Picard and his use of an Iconian probe that the diamond ship exploded after uploading an Iconian computer virus. (TNG - Mirror Universe novella: The Worst of Both Worlds)