Species 8472 is the apex of biological evolution. First encountered, and indeed named, by the Borg, Species 8472 exists within a parallel universe of fluidic space.
Threatened by Borg assimilation, species 8472 showed that resistance is never futile, and after fighting off the Borg invasion, they passed into our universe whereupon they destroyed millions of Borg drones, hundreds of planets, countless cubes and threatened the existence of all sentient life.
At least two metres tall when fully extended, the creatures are extremely flexible and incredibly strong; able to easily penetrate forcefields and ship bulkheads, capable of knocking their enemies about with precision and ease.
Among many abilities, the species display advanced telepathy and an ability to survive in a vacuum. Existing as one with their ships, which are themselves made from the same biological structure as Species 8472, both are able to regenerate themselves at will.
Temporarily defeated by the crew of Voyager and forced back into fluidic space, they remain a species best avoided.