Idumea is a minor Forge World of the Adeptus Mechanicus located in the Segmentum Obscurus, located near the edge of the Warp rift known as the Hazeroth Abyss, in the Calixis Sector. A malevolent Xanthite splinter faction of the Inquisition known as the Phaenonites, who are reviled even by their fellow Radicals, have made inroads into the governance of Idumea.
The Phaenonite infiltration of Idumea and its activities on nearby worlds have caused the Phaenonites to encounter an unforeseen foe in the form of the Amaranthine Syndicate.
This Calixian world has long been disadvantaged by its location on the edge of the Hazeroth Abyss and lack of strategic importance. The Forge Masters of Idumea have for many Terran centuries considered themselves to be sidelined and undermined by the central authorities of the Lathe System, a disaffection the Phaenonites have been quick to exploit.
Much of Idumea's upper echelons were hopelessly entrapped by the Phaenonite conspiracy, corrupted by the offer of forbidden lore and coerced through a concentrated program of targeted murders and sabotage. The Forge World itself now houses numerous secret facilities for the faction, as well as serving to provide a steady supply of manpower, experimental subjects, and a hidden port.
The Phaenonites have encountered a dangerous foe in the form of the Amaranthine Syndicate -- a deadly mercenary alliance of far traders and smugglers who operate within the deeps of the Hazeroth Abyss. At first, they seemed an ideal target for the Phaenonites' attention. However, as they moved against this group, it quickly became apparent that they had made a grave error. The Syndicate proved already to be the cat's-paw of another, darker force more than able to take on and destroy the Phaenonite agents with almost contemptuous ease.
Rather than fight a war against a power whose nature and extent they did not fully understand, the Phaenonites are now giving the Syndicate a wide berth where they can until they can find out more. This reversal has placed them in an invidious position, and so now they must thread a tightrope of bringing other, more legitimate Inquisitorial authorities to bear on what is a clear threat not only to them but to the sector itself, and do so without exposing their own true nature in the process.
- Dark Heresy: The Radical's Handbook (RPG), "The Infiltration of Idumea," pg. 122