Vigilus

Vigilus is an Imperial Hive World that represents the terminus of a key route through the Great Rift into the Imperium Nihilus. As a result, Vigilus has become a heavily contested strategic location and the source of conflict between several of the different starfaring species of the Milky Way Galaxy in the early 42nd Millennium.

At present, xenos forces, including Genestealer Cults, Orks and the forces of the Craftworld Aeldari have all launched assaults on the world. Its Imperial defenders stand on the brink of total collapse unless aid arrives soon. As a result, Vigilus has also been designated a War World by the Imperium.

History
Vigilus is a planetary asset deemed vital to the Imperium of Man's continued survival. Situated on the far side of the Nachmund Gauntlet -- the only stable Warp route through the dreaded Cicatrix Maledictum – the strategic importance of Vigilus to the Imperium cannot be overstated.

Vigilus comprises a number of continent-sized hive city sprawls that, in recent years, have become filled to overflowing with refugees seeking to escape the horrors besetting the worlds of the Imperium Nihilus beyond its aegis. Each such sprawl is protected by psychically charged Bastion-class force fields that render comatose any who would seek to breach their boundaries, enabling the defenders to slaughter them at leisure.

These force fields proved so effective that even when an Ork WAAAGH! smashed into Vigilus, the Greenskins were unable to assail the hivesprawls beyond, so took to seeking entertainment elsewhere by engaging in violent races across the sweeping open plains that separate the vast hivesprawls.

However, since the Great Rift tore across the galaxy, these Bastion-class force fields began to experience glitches, and eventually collapsed entirely to leave the hivesprawls open to attack. To make matters worse, the tide of Orks that poured past the inactive force field networks also had other, unforeseen consequences...

The Claw of the Thirsting Wyrm
For many Terran years, a Genestealer Cult calling itself the Claw of the Thirsting Wyrm had been thriving in the shadows of Vigilus. Having taken root in the subterranean depths below the militarised reservoir known as Greigan Hollow, many of their number had never seen the sky before their bloody uprising brought them to the planet's surface. Unbeknownst to the Vigilite defenders, the threat posed by the Ork WAAAGH! that attacked the world's hivesprawls had directly forced the cult's hand, for their long-planned time to rise was still a few solar months off.

Despite the initial successes achieved by roving Kill-teams of Adeptus Mechanicus Skitarii sent to counter these new attacks, every attempt to track the Genestealer Cultist threat to its source met with failure. Assailed by xenos from without and within, the Aquiliarian Council that governed Vigilus sent out a desperate call for aid -- unless the Genestealer Cults could be eliminated, Vigilus was as good as doomed.

Strike Force Icepelt
A strike force from Ragnar Blackmane's Great Company of Space Wolves Space Marines, led by the Primaris Space Marine Battle Leader Haldor Icepelt, had been thrown wildly off course as it travelled to reinforce the beleaguered Imperial armies battling the Chaos forces rampaging across the Stygius Sector.

Upon receiving the distress signal from Vigilus, Icepelt immediately ordered his ship, Wind of Fimnir, to change course. The Battle Leader was acutely aware that, should Vigilus fall, any retreat from the Stygius Sector could become impossible. Upon arrival, the Space Wolves vessel forced a passage through the flotilla of Ork spacecraft orbiting Vigilus and Strike Force Icepelt made planetfall.

By relying on their incredibly acute senses -- a trait of the Canis Helix unique to the gene-seed of the Space Wolves Chapter -- and the instinctive hunting skills inherited by those born of the world of Fenris, the Space Wolves were able to achieve results that even the most advanced technology of the Adeptus Mechanicus had failed to deliver.

Within only a matter of solar days, the Space Wolves were on the trail of the Genestealer Cult that had eluded the defenders of Vigilus for so many solar months. But it was only when Icepelt led his warriors into the ruined cityscape of Greigan Hollow that the full extent of the Genestealer Cults menace was revealed and the true battle for the survival of Vigilus began...

Aeldari Assault
Vigulus also became a traget for the various Aeldari factions. Vigilus had become a hub for many refugees seeking to traverse the Nachmund Gauntlet. The Drukhari's ruling Kabal of the Black Heart, who had annihilated whole flotillas of these civilian spacecraft, began spiriting their captured human cargo back to Commorragh.

Yet another threat confronted Vigilus when the forces of the Craftworld Aeldari of Saim-Hann appeared on the planet on an unknown mission. They engaged in a battle with a force of Primaris Space Marines drawn from the Ultramarines Chapter.

Canon Conflict
Though the Warhammer Community site has stated that the Genestealer Cult known as the Claw of the Thirsting Wyrm are responsible for the primary uprising on Vigilus and they have fought the Skitarii, in the Devoted Sons sourcebook for the 2018 edition of the Kill Team skirmish game, the back cover states said that Cult of the Pauper Princes is the source of the Genestealer Cult rising. This may be either a simple mistake or a representation of the reality that there is more than one Genestealer Cult on Vigilus.

Source