The Psiker Prophets
The Psikers are an ancient race, stemming back to when Earth was first expanding and encountered their first alien race. The alien empire quickly threatened the fledgling Imperium. In order to combat this new threat, humanity created a new species.
This species was an adhorrent abomination of mechanical and heavily genetically modified flesh. Their brains were more artificial intelligence than living tissue. This new species was created to help humanity by analyzing and then predicting the alien threat's next moves.
This worked too well, and once the alien threat was abolished the Psikers were sent into hibernation for hundreds of years for fear they would overtake humanity. After the Imperium's golden age of expansion however, they were re-awakened and sent to the world of Illusuar in the outer Imperium to help quell some of the rebellious systems.
Once again, they proved their worth, quickly taking control over systems. Because of this, they were granted the luxury of ruling over the star cluster they had conquered. With this freedom the Psikers went on to improve their species intelligence, warping their appearance and culture into something far alien to humanity.
They predicted the Great Collapse and were prepared for it. Theirs was one of the first empires to emerge from it. They immediately put into action hundreds of year-old plans for the Fleet Finale. A last-ditch effort to save the galaxy from impending doom, though not even they know what is the cause of the threat.
Soon however, the Psiker leadership fractured as they grew jealous of each other. Many sub factions of the Psikers now claim the title of Fleet Finale as they war amongst each other for power and the glory of saving a galaxy, a prophecy they will never fulfill.