Vigilus

Vigilus is an Imperial Hive World located in Segmentum Obscurus. It is now the focal point of a massive free-for-all as the Imperial Forces there try to keep the planet from the hands of Orks and Eldar invaders from without, and an insidious Genestealer Cult trying to subvert the planetary defenses from within. And then the Herald of Abaddon the Despoiler decides to come a-knocking...

About the Planet Itself

Prior to the opening of the Great Rift, Vigilus' main claim to fame --

and indeed its main planetary export -- was force field technology, specifically bastion-class fields, which were powerful enough to protect entire hive cities.  Its own cities are enveloped by these fields, increasing the world's defensive capabilities.

Both the Adeptus Mechanicus and the Ecclesiarchy have a significant presence on the planet, the former due to an ancient treaty signed with the ancestors of the current ruling dynasty, and the latter heavily influencing the local PDF (named the Vigilant) as well as the Imperial Guard garrison. Not surprisingly, there are a lot of Sisters of Battle around to enforce the Ecclesiarchy's will.

Not surprisingly, the nobility, the Mechanicum, and the Ecclesiarchy don't get along too well, but have stopped short of outright civil war thanks to the efforts of the Aquiliarian Council, which is made up of more reasonable members of the three factions.

Then, The Great Rift Happens

With the Fall of Cadia, Vigilus finds itself stuck on the north side of theGreat Rift, in the Dark Imperium. What's more, the world's strategic significance rises exponentially when everyone realizes that it's located at the end of one of two known stable routes across the rift: the Nachmund Gauntlet.

It's hives still protected by the psychic charged bastion fields, the world had become a staging area for any Imperial expedition daring to brave a foray into Imperium Nihilus (indeed, the survivors of the Stygies Front were using it as a rallying point), and the final safe harbor for refugees trying get out of it.

No One Expects the Orks

The first major threat to hit the planet in the wake of the Great Rift opening was a massive Waaagh! of Orks. Bursting from the Rift seemingly at random, and led by powerful Speed Freeks, the Ork fleet easily broke through the naval cordon and landed its boyz in the deserts between the great hives. While most of the fleet crashed and burned,