The worlds of Argoth, Rheelas and Kaggeran were, during the time of the Jericho Sector, collectively known as the Arkalas Cluster, and existed close together within a pocket of relatively stable Warpspace.
During the Age of Shadow, their proximity and the relative safety of interstellar voyages between them allowed the worlds to remain in contact with, and in support of each other using only relatively crude Warp-capable vessels.
It should come as little surprise, then, that the worlds slowly united into a single polity, as their fates were inexorably linked together.
According to the limited observations released by the Deathwatch during the planning of Operation Hammerfall which initiated the return of the Imperium to the Jericho Reach, the worlds of the Pelegiath Cluster collectively existed under the rule of a political body known as The Unity from approximately the 36th Millennium onwards, forming a tiny stellar pocket empire not dissimilar to those encountered by the Imperium during the Great Crusade.
However, unlike a few other enduring worlds within the Jericho Reach, such as Karlack and Castobel, those governed by The Unity did not maintain a properly Imperial government, nor did they retain their faith in the God-Emperor, forcing considerable corrective action on the part of Adeptus Administratum and Adeptus Ministorum elements of the Achilus Crusade to re-establish Imperial rule.
This matter was complicated by actions taken by The Unity to ally with the T'au Empire, a matter that would surely have resulted in even greater expansion of the territory claimed by the T'au within the Reach had the Achilus Crusade not swiftly annexed these worlds.
Ebongrave has determined to let the populations of the cluster die off and then be replaced by what he considers loyal Imperial settlers.
- Deathwatch: The Jericho Reach (RPG), pg. 64