a long time after the horus heresy and the defection of the Thousand Sons the Adeptus Astartes needed a new legion of those who were psychically inclined... but the Thousand Sons had been corrupted by chaos magics and the ruinous powers. How would they be any different from those who had been the best psychics of all??
The answer was simple, geneseed taken from one of the most loyal legions and leadership from a great librarian would make a stalwart company of soldiers to aid the Space Marines with their mastery of telekinesis and mental power and serve the Emperor to the death.