Marius VII. Imperial Hive World. Located within the Romanov Sector of the Segmentum Obscurus, Marius VII lies slightly to the southeast of Cadia and the Eye of Terror. The Hive World began as a mining planet, but quickly developed, like many planets near the Cadian Gate, into a military outpost for fielding troops that would eventually serve in warding off the hordes of the Ruinous Powers. Marius VII was also distinguished by being the base of an Inquisitorial Conclave, assembled to aid in containing the powers of Chaos within the Eye of Terror. Marius VII was not a fortress as much as it was an assembly, where raw recruits might be delivered for training, and military materiel could be distributed.
Without warning, the skies rained fire upon Marius VII as a battlefleet of Chaos warships unleashed hell upon the unsuspecting planet. Following the apocalyptic bombardment came the minions of the Ruinous Powers; the Chaos Space Marines of the Iron Warriors Legion, superhuman warriors and fallen champions of the Imperium now sworn to its destruction; the endless hordes of traitor Guardsmen, mutants and heretics that are known as the Lost and the Damned; the incarnations of the will of the Dark Gods, the terrifying Daemons of Chaos. All these horrors descended upon Marius VII with an inhuman fury and perverse glee, slaughtering all that they could find. The Planetary Defence Force did they best they could, fortifying parts of the planet that could be easily defended, and evacuating what civilians they could into those areas, but most of the planet was consumed by the fires of Chaos in a matter of weeks, with the small strongholds bitterly holding on.
And then, hope descended upon the shattered world as the combined might of the Imperium descended upon Marius VII with a vengeance. The armoured might of several companies from the Imperial Fists Chapter of Space Marines, masters of siege warfare, led the way, followed by a Cadian Battlegroup consisting of the 257th, 132nd, 79th, and 49th Shock Trooper Regiments, as well as the 18th and 43rd Armoured Regiments and the 13th Heavy Tank Company. Several Inquisitor Lords, of both the Ordos Malleus and Hereticus, followed with their Chambers Militant, the Grey Knights and Sisters of Battle; the Daemonhunters led the battles against the the daemonic powers aiding the Ruinous Powers, while the Witch Hunters immediately began to purge any traitors they found.
As the combined might of the Emperor’s warriors descended upon the stricken Marius VII, their first priority was to aid the faltering PDF and strengthen the Imperium’s remaining strongholds on the planet. The first counterattacks shattered the momentum of the Chaos offensive, and the Imperium’s tenuous grip on the planet tightened. However, the Iron Warriors were the chief rivals of the Imperial Fists in the matters of siege warfare during the Great Crusade, and became their most bitter foes during the Horus Heresy. Both sides dug into the endless concrete of Marius VII, and a long, bitter war began as the many heroes and legions of both sides flocked to the standards of battle…
That there be the premise, essentially. I'll come up with some geography for it in a little bit, too :)