It hosted considerable natural resources in the form of sub-strata promethium [grade Vulcan-989] and other geologically attendant chemicals, exploitation of which is fully established and tithed to the consumption of the Achilus Crusade via Departmento Munitorum edict.
The planet was overrun by the Tyranids of Hive Fleet Dagon, which crushed all Imperial resistance planetside. The primary starports and habitation zones were lost, the mainline Astra Militarum garrison effectively destroyed along with lunar governance. Prior to the arrival of a Deathwatch Kill-team, intelligence estimated population causalities were already 89% and rising.
In the colony's final days, catalytic organisms became environmentally prevalent in the Tantalus equatorial zone -- it was estimated at the time that only twenty to seventy solar hours remained until biospheric conversion began. Fragmented resistance held out in the polar regions.
However, the armament and size of the remaining Imperial forces were totally inadequate to prevent the Tyranid swarm from systematically advancing to scour the world of all life.
The crusade forces had used Tantalus as a "second line" rotation post for the Orpheus Salient warzone. Indeed, median garrison levels were approximately 10,000-15,000 troops on rotation.
Additional forces of the Adeptus Munitorum provost command overseer/disciplinary corps were charged with maintaining order and ensuring production, and comprised roughly 200,000 operatives.
Tantalus is a world approximately two thirds the mass of Holy Terra in stable orbit around a Type III gas giant, whose length of day, gravity, and climatic conditions point to deliberate alteration during the High Dark Age, and was an Agri-world serving the primary Castobel Hive World during the period of established Imperial rule of the Jericho Sector.
Over the last millennia while out of Imperial control, Tantalus had undergone a transition towards a cooler climate associated with the slow onset of a rotational ice age and prevalent conditions on the moon were now predominately stormy, its topography succumbing to sub-arctic/tundra with polar ice caps advancing to cover 32% of the moon's surface.
However, with the coming of the Achilus Crusade and the resurgence of Castobel I, extensive promethium refinery infrastructure, the starport/city of "Tantalus Landing" and other settlements have been constructed and the population has trebled through forced immigration of tithed workers and crusade forces.
For more than five solar months, the Castobel System came under the assault of the all-devouring Tyranid menace, and Tantalus was no exception.
While the fate of the Hive World that gives this star system its name still hangs in the balance, Tantalus has not been so fortunate, and the outlying Imperial colony is now in its final death throes.
With the battle for Tantalus lost, and the jaws of the Tyranids closing inexorably around this moon, the few survivors that remained were desperately caught between a doomed struggle to survive and desperate hopes for escape. However, the lives of all that remain could now be measured in no more than solar hours.
The fall of Tantalus and the horrific deaths of its inhabitants may not have been entirely in vain -- a small Adeptus Mechanicus Biologis contingent was present studying local life on Tantalus at the time of the first attack.
Stranded on Tantalus, they took the opportunity to turn their unflinching gaze upon the monstrous life forms that came to devour the world.
Their priceless data on the bio-forms and attack patterns of Hive Fleet Dagon, bought at the sacrifice of most of the magi's own lives, may yet prove invaluable to the war in the Orpheus Salient and must be claimed by the Deathwatch, whatever the cost.
For Tantalus, the end has come very abruptly. The battle was tipped in the Tyranids' favour by the appearance of one of their bio-ships bringing heavy reinforcements in orbit around the industrial moon.
As fighting against the vanguard organisms was already desperate below, the bio-ship was able to encroach almost unopposed, and unleashed wave after wave of spore pods heavy with monstrous predator-forms and lethal parasites which have already slaughtered the last serious Imperial resistance and begun to poison and consume Tantalus' biosphere.
Unable to break through the Tyranid assault and escape off-world, the shuttle carrying the last member of the Mechanicus team, Magos Vyakai, fled for Tantalus' southern polar regions that had not been overran thus far.
Unfortunately, it was brought down by the Tyranids a few dozen kilometres short of the relative safety of the polar ice fields, crashing near a promethium extraction complex designated Pyroclast-Gamma-9.
Since the shuttle's crash landing, fragmented transmissions from the magos indicated both his personal survival and that of the vital datacore he was carrying.
The Deathwatch frigate Thunder's Word that had been due to rendezvous with the escape shuttle is now scrambling a last minute extraction mission; a Kill-team is to be deployed immediately to Pyroclast Gamma-9.
The last transmission was three solar hours before contact was lost in the static storm. This communication indicated that vanguard Tyranid predators had been sighted and the great devouring swarm could not be far behind.
Pyroclast-Gamma-9 was an industrial facility and promethium refinery in Tantalus' extreme north, set amid sharp-edged rocks, tumbledown crags and coarse tundra.
One of a string of similar facilities scattered across these latitudes, it was a sprawling complex, with numerous buildings, stations, and supply dumps spanning several square kilometres in size, although much of its uninhabited infrastructure (such as the chemical storage tanks) was actually buried deep underground beneath the steel domes, pipe-farms and dull ferrocrete blockhouses that make up the facility.
During full operations, the area was staffed by a crew of Departmento Munitorum functionaries and technomats, their attendant servitors and a sizable workforce of convict labour kept in check through the use of pain-inducing punishment collars and the overseer's lash.
Unfortunately, the crisis had seen tragedy and death come to the area. In the days before Tantalus' fall, vanguard organisms of the Tyranid swarm had attacked, and many of the technomats had already fled or been killed, leaving the prisoners out of control and the station's servitors on emergency protection programming, but without any form of central control.
Hormagaunts and Termagants already lurked in the shadows of the nearby ravines and had forced their way within several of the buildings, butchering everyone within, while the dark-winged shapes of Shrikes arced across the cold skies in increasing numbers. The swarm was on its way, and Pyroclast-Gamma-9's existence could now be measured in solar hours.
The area around Pyroclast-Gamma-9 was sub-arctic tundra, and the terrain was riven with spars of granite, broken ground, steep crags, scree hills, and muddy sinkholes worsened by industrial traffic and chemical pollution. The air was cold and thick, heavy with the stench of burning chemicals and drifts of black smoke.
The light levels were low -- noon on Tantalus corresponded to a bright twilight on most other Human-habitable worlds. While the skies to the polar north were a harsh icy grey, the southern horizon had already been swallowed by a rolling purple-black maelstrom, shot through with unnatural veins of cankerous yellow -- the atmosphere of the moon itself was being changed and devoured by the Tyranid spores.
Dotted around this bleak natural landscape were the obvious signs of the Imperium's hand in exploiting the moon's resources in the shape of metal grillwork pathways, strings of battered lumen globes, machine workings, and the brutal scars of past excavations.
The tracks of heavy vehicles had churned up the ground. The architecture in evidence was brutally functional and prefabricated in origin, constructed of harsh square-sided ferrocrete bunkers stamped with codesigns and Departmento Munitorum admissions to workers to obey and remain productive. Looming iron-black engines and rusted pipe-work crisscrossed the landscape.
Here and there were signs of then-recent violence and destruction, machinery overturned, areas blackened by fire (some still smoking) and the grey bodies of dead convict-workers. Many had been stripped of their flesh by the claws and fangs of Tyranid vanguard organisms, but others bore evidence of gunshot or stab wounds caused by other Humans.
Tantalus had almost succumbed to the Tyranids' shadow. Vox communication and auspex tracing on its surface had become almost impossible. Teleport access was also impossible, and any craft that approached the moon's near orbit and upper atmosphere were subject to swift attack and destruction by the Tyranids.
A single Drop Pod fired in from high orbit had the best chance of evading detection, and that's where the Deathwatch Kill-team came in to begin the search for Magos Vyakai and his precious data.
- Deathwatch: Core Rulebook (RPG), pp. 378-380, 384