The opening of the Great Rift created an open wound of madness in the skies Calibanis-III. The Death World, once a thriving colony world, was hit long ago by a catastrophic Warp event known as the Great Devastation. The event mutated the ecosystem, forcing the once-tame jungles to grow out of control and the wildlife to mutate into bloodthirsty monsters. The humans who survived were forced to build fortified cities and fight for survival on a daily basis. With the return of the Imperium during the Great Crusade, the world was mostly retaken, but the dangers have never lessened. A Fifth Founding Space Marine Chapter from the Whitescars bloodline known as the Stormwatchers were gifted it as their Chapter World after their deeds in the Rosh Crusades against a massive Ork WAAAGH and then volunteering to oversee and protect the Galan Collective, a group of loyal sectors on the Imperium's outer edge alongside several Imperial Guard regiments.
Now, the light of the Astronomican has gone out for the sectors of the Galan Collective, and the astropaths of those worlds closest to the Great Rift were lost either to agonizing deaths or daemonic possession. Those who survived on further flung worlds suffered the same or worse fates after trying to send out messages for help. They don't know they are now in the darkness that is Imperium Nihilus. On Calibanis-III, things get much worse with the arrival of a splinter fleet of Hive Fleet Leviathan. The Death World itself goes into a war for survival as the Great Devourer comes to turn every inch of life into biomatter to feed its never-ending hunger.
The Chapter Fleet attempts to intercept the Tyranids before they get into range to start dropping biopods onto the surface while on the surface, Imperial forces are already fighting as the Genestealer cult that has been taking root within the planet's population for centuries. The cult executed attacks on critical installations, going as far as blowing holes in the walls and destroying wall defenses to allow the world's deadly fauna into the fortress cities. In orbit, the Imperial and Tyranid fleets meet. The Imperial Fleet is destroyed down to the last ship, and the true invasion of the world begins. The Stormwatchers and the Planetary Defense Force fight a war on multiple fronts that few, if any, believe they have a chance of victory. None of them know that their message has been heard and help is coming in the form of lethal dread and wrathful shadows.