The Harvest Cluster is a federation of non-Imperial human worlds pre-dating the Great Crusade. Their distance from the core worlds meant that the Horus Heresy halted the Great Crusade long before it reached that far into the Eastern Fringe. Attempts to bring the Cluster into the fold have been failures, as the humans there are armed with superior technology and heretical knowledge of its workings, having never been exposed to the dogmas of Mars. They consort with Xenos willingly, and have earned the ire of the entire sector's human population. However, the resources required to exterminate them have been better used elsewhere, as the federation seemingly has no desire to expand, and has not infringed on Imperial space since the original conquest of the Gulf.
Rumour has it that they are employing an ork freeboota fleet composed of the survivors of Boltbrainz' Waaagh after they fled Ravenforge.