The Nephilim were a race of pseudo-cybernetic insectoid beings from an another galaxy or other unexplored region of the universe. The first claimed sighting of the Nephilim that is known occurred in 2374. Later alleged sightings were reported all over the Alpha and Beta Quadrants. (The Devastating Discoveries)
Prior to the 2380s, sometime after the end of the Dominion War, the Nephilim were unknown to every race in the galaxy. However, there were accounts that the Nephilim were been in the galaxy, starting from at least early in the 2370s.
In 2374, one of the crew of the USS Voyager mentioned that a ship not found in the databases appeared in the long-range sensors. Upon investigating it, the ship did not attacked the Voyager, but departed. The ship was described as a twelve-legged black spider with seemingly part organic with veined, tentacular support trusses, shell like structures, and six devilish wings.
Sometime in 2381, the USS Enterprise, received a distress signal coming from a ship being attacked by such ships. The Federation agreed that it sounded like the Nephilim and believe them to be the race mentioned in the reports.
Two years later, a Cardassian Valor-class cruiser encountered a Nephilim ship and was disabled. The USS Sathanas was sent to investigate, only to be under attacked as well. After that incident, it attacked the USS Violet Baudelaire, but it was chased away.
The Nephilim were responsible for destroying countless races, and the domination of many galaxies in the universe in thirst of power beyond comprehension, hence the reason why it is a real threat posed to the Federation even the Borg and the Dominion. The Nephilim were also the one who instigated the supernova at the Romulan system and destroyed their homeworld in 2387. Other supernovae they might have instigated included the Triangulan and the Horologian supernovae.
In the Hebrew Bible, the Nephilim are beings mentioned twice in Genesis 6:4 and Numbers 13:33. Its traditions are also found in a number of other Jewish and Christian writings.