Ghosar Quintus is an Imperial Mining World of the Ultima Segmentum. It was the place where the Imperium of Man discovered the first major infestation of a human planet by a Genestealer Cult psychically calling out to the Tyranid Hive Fleets, though they were still several solar decades away from the Milky Way Galaxy.
Ghosar Quintus was considered a primary example of a "Delverworld," a planetoid settled by the Imperium that is given over completely to mining operations. Delverworlds are particularly common in the Eastern Fringe of the galaxy.
Ghosar Quintus was ruled by the noble family known as the Trysst Dynasty. Before the hideous truth of Ghosar Quintus was revealed, it was used as a prime exemplar of a "Delverworld," a planetoid given over entirely to mining the Eastern Fringe.
Its tithes were consistently at least a fifth higher than the required level, and the adamantium mined from its substrata was purer than any other source in the sector -- every ingot polished to a bright sheen, neatly stacked and dutifully categorised by countless Trysst workers. More than that, the dynasty's workers were content to subsist only on gruels and nutrient pills, and there were no recorded incidences of unrest.
To the Imperium they seemed a model population. When this came to light through the planet's tithe reports and later Lexmechanic assessment, the Departmento Munitorum shipped out work groups of the Trysst Dynasty to every other Delverworld within reach, there to act as consultants and role models for the other, less efficient populations to aspire to.
But in truth, Ghosar Quintus was home to a Genestealer Cult, the infamous Cult of the Four-Armed Emperor, which used its model reputation and the efficiency of its workforce to spread its version of the Genestealer Cult across half the galaxy.
Cult of the Four-Armed Emperor
The first confirmed Imperial military engagement with a Genestealer Cult occurred upon the mining colony of Ghosar Quintus in 680.M41. Investigating what appeared to be a perversion of the Imperial Creed, Inquisitor Chaegryn led a team of Tempestus Scions to Ghosar Quintus and ventured into the depths of the Great Pit, the greatest mining complex present on the world. The deeper Chaegryn ventured, the more evidence of deviance he found. Strangely, his last communiqué stated that all was well, and that the Trysst Dynasty that ruled Ghosar Quintus should be left to its own devices.
It was a full Terran year before Chaegryn's fellows in the Holy Ordos of the Inquisition noticed that something was deeply wrong. A five-man Kill-team of Deathwatch Space Marines known as Kill-team Excis was sent on a follow-up mission of lethal investigation, yet they too were swallowed by the mysteries of the Great Pit, apparently slain in their first encounter with the Genestealers that lurked within the planet's depths. Only when the steel-willed Chaplain Ortan Cassius of the Ultramarines mustered his own hand-picked Kill-team was the vile truth unearthed -- Ghosar Quintus was home to a xenos infestation.
Kill-team Cassius fought through hundreds of Genestealer Cultists as they plumbed the depths of the Trysst Dynasty's corrupted world. Though they made it out alive, the Space Marines were changed forever by the gruesome ordeal. Most shocking of all was not the Genestealer Patriarch that lurked at the heart of the cult, but the damning evidence in the mining cult's shipping holographs.
Under the guise of industry, the xenos-tainted Trysst Dynasty had spread its curse across not only the Ghosar System, but throughout the sector and perhaps across half the galaxy. The implications were staggering; the scale of the xenos threat was unprecedented.
Only when the First Tyrannic War began several solar decades later after the arrival of Hive Fleet Behemoth did it become clear that Genestealer infestations represented only the first step in the Great Devourer's plan to consume the galaxy.
- Codex: Genestealer Cults (8th Edition), pp. 20-21
- Codex: Genestealer Cults (7th Edition) (Digital Edition), "Ghosar Quintus"
- Deathwatch: Overkill Rulebook (Board Game), pp. 51-52
- Deathwatch: Cepheus (Short Story) by Braden Campbell
- White Dwarf (September 2016), "Deathwatch Overkill - Kill Team Excis"