The Belkans are the revitalised and much nastier incarnation of the Belkan Imperium.
Though the original Imperium was destroyed by the UEF and the Tanaran Empire before the UEF's jump to the New Regions, there was a spectre lurking in the shadows. The Battle of Hell left two million Belkans and two million Rey't to burn as the planet Hell turned itself inside out. By a factor of impossibility, they survived the ordeal, but their mutilation affected their minds as well as their bodies. With what little the UEF had not reclaimed and fled the planet with, the remainder of the Belkan force were able to stay alive long enough for the last of the Belkan navy (a few battered cruisers and a damaged dreadnought) to wander past and pick up their distress call.
What the Belkan navy was unaware of was that their rescuees had been changed into something barbarous. Upon arrival on the ships, the rescuees, toughened by their ordeals and their pain, tore apart the crew, and began experiments to revive and reanimate them as ones of their own. Disturbingly, they were successful.
After pirate-like raiding parties which increased their ranks and gave them resources, the Belkans had amassed a small fleet of stolen vessels. In the shadows they lurked, scavenging technology, shipwrecks, anything they could get without being discovered by the Tanaran Empire or another of the Great Houses of the galactic council. A long time later, they reclaimed their ravaged world of Belka Prime, which had never been rebuilt after the Tanarans bombarded it to ashes.
Things were quiet for a few years while the Belkans advanced their technology and increased their numbers in secret. Only twisted minds could come up with the technology they did.
So it was, the Belkans arose once more from the fires of Hell and began a firestorm of the galaxy. Using a living machine technology to tear the very resource and goodness from the planets they assaulted, from crust to core. They drew up everything from the planets and used it to produce an ever more ambitious fleet.