Callamus-
Forgeworld Callamus is a young one for its designation and a highly unusual one at that. Callamus boasts little in the way of particularly advanced technologies save for redeveloping the exo-lance* and advanced las weaponry in general and highly efficient industrial technology. Callamus is notable for a few things. First and foremost are its ability to mass produce extremely large amount of relatively advanced weapons, equipment, ships, and any other kind of products needed (outstripping many Forgeworlds whom have been established for millenias) and its high level of militancy. Callamus possesses multiple large warfleets and armies including multiple titan legions which are often dispatched far afield to destroy threats to the Imperium and Callamus. Millions of Callamus soldiers have fought and died far afield fighting such threats as Tyranids, Chaos, Orks, Eldar and Dark Eldar. Callamus itself also boasts defenses that rival Segmentum fortresses.
Callamus is somewhat of an oddball among forgeworlds in that its menials do not labor in harsh and miserable conditions. Instead each are afforded near extravagant level of luxury compared to most forgeworld laborers. But this is paired with an extensive police state which pervades the entirety of the population. Menials live in luxury but are under constant surveillance, most cult activities are caught and eliminated in their infancy. The only extensive cult in record was discovered among the techpriests themselves and also eliminated with extreme prejudice.
Another odd note with Callamus is that the organists influence is high in Callamus, with the menial population being genetically enhanced over generations gradually creating superior humans which translate to improved tech priests in the future as hopefuls are inducted into the Machine Cult.
After death of the Emperor and the splintering of the Imperium Callamus has taken control of the Karatus sector using its vast influence and continues to expand, putting down xenos invasion and Chaos and Abomination cults with high efficiency.
Callamus also has a benefit in that the three forgeworlds near it are quite the oddballs themselves each with their own specialties, forgeworld Callamus has cultivated good relations with them as much as possible.
Lainshul- A forgeworld established on a water world, focuses on aquatic technology and terraforming.
Tempus-Sigma-Ten- A 'forgeworld' that was originally a naval base prior to the Horus Heresy, production capability remains low but has extensive ship based technology, with rumors indicating they possess the blueprints to the Gloriana class superbattleships.
Amkarius- A forgeworld formed by a lost Adeptus Mechanicus expedition whom had the misfortune to collide with a space hulk within the Warp. After painstakingly clearing the Hulk and bringing its various systems under control and extensively modifying the hulk into a proper vessel the expedition arrived thousands of years after it had set out to the planet Amkarius which had already been colonized by the Imperium. Suspicions were high and there were moves towards destroying the remnant of the expedition, but the tech priests reportedly bought the support of Mars by providing numerous technologies which had been lost since the Horus Heresy. Rumors indicate that Amkarius still holds a treasure trove of technology in reserve.
On the topic of technology and forgeworlds, before the fall of the Imperium Callamus has traded for technology heavily with various forgeworlds.
*: Think a giant fuck off lance that can one shot a Grand Cruiser if your lucky, and heavily damage battleships as well.
Note: Callamus also possesses advanced VIs which is based on a Dark Age of Tech VI/limited AI named Immos recovered from an ancient human facility.
Note 2: Callamus has enough industrial capability to, theoretically, replace every single one of its warfleets within a few years.