The planet Proditorem, Sector Geminus, Ultima Segmentum. A hive world with a population of 100 billion, or it did, before the cultist uprisings. Over the course of several months this hive world was overrun with Chaos uprisings. Cults are common enough within the region know as the Trail of Dust but cultists rarely work together as well as they seemed to at the beginning of the uprisings. The PDF and arbites could do nothing as they came up from the lower hives and pushed well into the mid-hive when their momentum began to slow. Smelling blood in the water pirates, warbands, and any who believed their was something to be gained began to come to the world to prey upon it in it's moment of weakness. This was shattered though when the Imperium at last got aid to the planet. Several regiments of Imperial Guard, an arbites task force, and rumors even abound of the Inquisition having a presence. The Imperium Navy was fast to act mere moments after leaving the warp torpedoes and strike craft were inbound on the vessels preying upon the world chasing them away and stranding some of the pirates and voidfarers on the world below. The task force while impressive was unable to pursue the chaos vessels beyond the orbit of Proditorem however and thus some now lurk in the asteroid field some distance away.
Even as the fleet secured orbit though the guard was not to be outshone landing light infantry in the upper hive to prepare defenses and setup command and control as well as in the mid-hive almost right on top of the cultists pushing up through the hives. Within the capitol hive of Celatum, they made sure to show a strong presence rapidly shattering and forcing any chaos aligned forces down deeper into the hive while securing almost half of the mid-hive in a few days. A wanted list for the most infamous individuals is now being circulated about with what information they have, the infamous pirate Brutus Lanius Nero and his crewmembers including Choirmaster Verdi and Archmilitant Celeste Davayos are actively pursued wanted by the arbites for a thousand crimes, the reviled Tzeentch sorcerer Embra Parasitif and his allies and cohorts such as Malignus Raevan are prime targets for a dozen Imperial administrations for their acts in the name of their dark god. The goal of the Imperium is to drive them out and kill them in the open. They know they are on the world and will not allow them or any other of the arch-heretics off the world.
Cut off from the upper hive, the outer hive ring, any docks, and any teleportariums the Imperium knows they and all the other cultists are now cut off, forced down in the mid-hive or lower hive it is only a matter of time until the noose closes as the Imperial forces slowly yet assuredly secure the hive one level at a time.