Borg

The Borg were a pseudo-species of cybernetic beings, or cyborgs, from the Delta Quadrant. No single individual truly existed within the Borg Collective (with the exception of the Borg Queen), as all Borg were linked into a hive mind. Their ultimate goal was the attainment of 'perfection' through the forcible assimilation of diverse sentient species, technologies, and knowledge. As a result, the Borg were among the most powerful and feared entities in the galaxy, without really being a true species at all.

The physiology of each Borg drone varied according to the species from which it was assimilated. Upon assimilation, a drone ceased to grow body hair and developed an ashen, grayish skin coloration, ignoring original skin pigmentation. Cybernetic implants were either surgically attached to the body or grown internally by nanoprobes injected into the bloodstream; in certain cases these implants could cause severe skin irritation. The nature of these implants varied from drone to drone depending on the drone's intended function, but basic nodes of interlink for communications with the Collective and a myo-neural cortical array to control movements were implemented in every drone. In most cases an eye would be replaced with an eyepiece that improved vision and an arm would be amputated altogether to make room for a functional prosthetic; in tactical drones a weapon would be included, and some drones had medical tools built in. Source: Memory Alpha

After the transwarp network was destroyed by the USS Voyager in 2378, the Borg was never seen again anywhere near the Alpha Quadrant.