The battle takes place on Attraignnor IV, an Imperial world out on the very fringes of Imperial space, near the Tau Empire. So far, the Tau have mostly left it alone. But being on the very frontier of Imperial territory, Attraignnor IV isn't particularly well protected or patrolled by Imperial forces, and it pretty much has to fend for itself. The planet has so far done pretty well for itself, with very little help from the Imperium.
Originally Attraignnor IV was colonized just for the sake of expansion, even though it didn't seem to be a particularly valuable planet in any way. But the people of Attraignnor IV were extremely successful at mining pockets of gas located deep within the planet. The gas is greatly valued for its use as fuel in a variety of vehicles, and inhabitants of Attraignnor IV made a lot of money exporting it. This naturally pleased the Imperium, which proceeded to tax them very heavily on their profits.
Resentment began to brew all over the planet because they had to pay so much of their hard earned money in taxes to the Imperium and get basically nothing in return. Gas mining is a very dangerous and risky enterprise, and colonizing a world in the first place is extremely difficult for the early settlers. So they felt as all their risk and hard work was being punished rather than rewarded by the Imperium that basically left them to fend for themselves.
The breaking point came when a particularly large and powerful Ork Waaargh, led by Warboss Gruka Spinetearer, attacked Attraignnor IV. The defenses were overwhelmed and the planet was quickly infested with rampaging Ork warbands. Planetary Governor Devonius Krell had sent out calls for assistance to the Imperial Navy, the Imperial Guard, and the Space Marines. The sub-sector's Naval and Imperial Guard forces did not want to provoke an attack from the nearby Tau Empire, especially with their limited forces so far from the core worlds of the Imperium. So the Navy and the Imperial Guard both replied that all of their forces were tied up and they had nothing available to send to help defend the planet.
The Space Marines of the Imperial Fists did respond, and sent a few squads led by Lord Captain Octavio Ferranus, Commander of the Fourth Company, Master of the Marches. When they arrived they did their best to strike at the most powerful of the rampaging warbands in the more vital areas of the planet. But Captain Ferranus realized how ridiculous the situation was. The planet was hopelessly under supported. It had no outside support. Most of the troops it had raised had been called away by the Imperial guard, leaving them with a token defense force. The few ships in the system fleet had been easily chased away by the large Ork fleet. The situation seemed hopeless.
Being far from the prying eyes of the Inquisition, Attraignnor IV had a relatively large number of chaos cultists who secretly prayed to the gods of chaos for all kinds of things. Now, in these desperate times, much more of the population were convinced by these cultists to turn to the chaos gods for salvation. Arch Heretic Fellonius Degrassio, Demagogue of the Cult of 36 Secrets, managed to gain a particularly massive following amongst the populace.
These citizens formed huge cultist militias to fight back against the Ork invaders. They fought with incredible zeal and determination, some even seeming to fight with superhuman powers granted by their gods. It was even rumored that some of the largest cults had succeeded in enacting dark rituals to summon forth daemons from the warp to attack the Orks.
The Imperial Fists Captain began hearing rumors of the militias, and how they may be connected to cults that might be connected to chaos worship. But as the tide began to turn against the Orks due to the ferocity of the cult militias, Captain Ferranus accepted this as necessary and allowable in order to achieve the greater good. He even began working with some of the more influential militia leaders, including Fellonius Degrassio, in order to coordinate attacks against the Orks.
By this point, Captain Ferranus had grown increasingly frustrated with the situation, as the casualties amongst his men continued to increase. Every day he would request aid: more marines, naval support, the Imperial Guard. He explained that the number of invaders was greater than anticipated, and the situation was hopeless unless someone came to their aid. But none would come. He grew angry at the inability of the Imperium to act. His anger even extended to his own Chapter, the Imperial Fists, who had left him to die in a pointless and impossible fight. As the war raged on, Captain Ferranus grew more and more determined to destroy the alien invaders, no matter the cost, just to spite those who had abandoned Attraignnor IV to its fate.
Then things suddenly took a turn. The Ork fleet that had been chasing the planet's small fleet of ships was engaged and scattered by a large Tau armada. The Tau, however, did not attack the planetary fleet or the planet itself, and simply left the system. With the Ork fleet gone, the system fleet was able to support the ground battles with naval bombardments and bomber attacks. Together, the planetary defense forces, the cult militias, and the remaining Imperial Fists managed to fight back and eventually destroy the remnants of the Ork warbands that had landed on their world. The war was over, but at great cost. Much of the planet was devastated, including the source of their trade income, and it would take years, possibly decades, for it to recover. Most of their mining facilities were destroyed, and in their place stood massive columns of fire from the burning gasses shooting up through the ground.
Then finally, once the war with the Orks was already well in hand, a small Imperial naval fleet arrived at the planet. Naturally, Governor Krell thought, it was just like the navy to show up with reinforcements once they were no longer needed. But when some of them came down to the planet, the governor was told that they weren't reinforcements. They were with the Imperial government, and there to collect the planetary tithes.
Devonius Krell was enraged. After being told there was not a single ship available to help save his planet, and being left on their own to fight against hopeless odds by the Imperium, a fleet had shown up to make sure they paid their taxes to the institution that had abandoned them. He was especially furious that they would show up now, when their planet was devastated and their financial situation at its most dire. He had had enough. His planet clearly had no need for the Imperium of Man, and they had proven it by surviving the crisis without help. The rebellion that had been building for years and years was now coming to a head.
In a fury, the governor had the tithe collectors arrested. He had his planetary defense guns and his fleet attack and destroy the unsuspecting Imperial fleet. There was no going back now. They were in open rebellion. Perhaps this would finally draw the attention of the Imperial military.
Shortly thereafter, as fate would have it, an emissary arrived from the Tau empire, trusting that the destruction of the Ork fleet was a suitable show of peaceful intent. The Governor needed to find a way to survive the retaliation that would surely come from the Imperium, and the Tau had already aided his planet more than the Imperium of Man ever had. So the Governor struck a deal, agreeing that his planet would join the Tau Empire and fall under their protection.
The population of the planet was already well aware of the rebellion of their world. After what they had been through, there were relatively few on the planet that still wished to remain loyal to the Imperium, but hearing the news that they were to join the Tau Empire split the population in two. Some knew that it meant the military protection they needed, and were so glad to be rid of the oppressive Imperial rule that they were ready to accept the ideals of the alien Tau.
But many others would never accept such a fate. They had just given everything they had and more in a desperate fight to stop an alien menace from conquering their world. There was no way they were going to turn around now and just hand the world over to a different alien menace without a fight. Especially those who had turned to the ruinous powers to aid them in the victory, and who wanted their world to be devoted to the praise of the chaos gods.
The entire planet erupted into a fierce civil war. On one side: those who supported the decision to join the Tau empire, loyal to Planetary Governor Devonius Krell. On the other: those who were against, some because they simply despised all xenos, some because they disagreed with the ideals of the Tau Empire, and some because they had seen the power of the chaos gods and wanted to devote the planet to their masters. These included large forces of cultist militias, many of them now lead by Arch Heretic Fellonius Degrassio, Demagogue of the Cult of 36 Secrets.
Captain Ferranus of the Iron Fists had been trained all his life to hate all xenos and all that they stand for. He had also been trained to hate Chaos, but by this point his exposure to it during the war against the Orks had affected him. He chose what he thought to be the lesser of two evils, and fought with the rebels against those loyal to the Planetary Governor. Soon, drawn to the planet by the whispers of the dark gods, small bands of Chaos Space Marines began arriving on the planet to join the fight. Many of them eventually banded together under the brilliant leadership of the Imperial Fists Captain.
Also, a renegade marine chapter with a large spy network was told of the rebellion of this world. In it's desire to aid any world rebelling against the Imperium, the chapter sent a contingent to the planet to aid in the fight, not knowing it had turned into a brutal civil war. War continued to ravage Attraignnor IV, with the capitol city being mostly destroyed by the constant bombardment. The remnants of the Planetary Government, including Planetary Governor Devonius Krell, are now hiding in bunkers deep beneath the capitol city while the war rages above them.
There are also in some places, including the capitol city, small pockets of men still loyal to the Imperium itself, caught between two warring armies of traitors and just trying to hold out and survive. This includes Chief Justice Frenauld Grennior of the Adeptus Arbites, who, with a small force of Adeptus Arbites, is holding out in the Arbites precint house in the capitol city. There are also some Imperial Guard troops still loyal to the Imperium left in the capitol city. Led by Lt Sgt Bartlmus Ortega, they are doing all they can just to defend their HQ from being taken by the various traitors. There are even rumors of a couple of Imperial Assassins operating somewhere in the capitol city, doing anything they can to punish the traitors.
It is at this time that a Tau army arrives on the planet. Not wanting to lose the planet they had been promised, the Tau send an army in to secure the capitol city of the planet and defend Governor Krell, with whom they made their deal. Little do they know that as they arrive, the forces of the unknown renegade marine chapter and the warband of the Imperial Fists captain are also converging on the capitol city to find and eliminate the last remnants of the loyalist government.
Fallen Imperial Fists:
With the Imperial Fists Commander came his command squad including an Epistolary Librarian named Santiago Botrejadre, as well as Chaplain Heinz Wartsmicht, three tactical squads, 2 assault squads, and one devastator squad. His force was further supported by Techmarine Ollivander Smythton who ministered rites to the force's 3 predators, bikes, 5 rhinos, and 3 land speeders. The brutal battles with the vile Orks and the subsequent civil war has reduced and scattered the Imperial Fists' forces and squads have been forced to form from the mixed survivors. Chaplain Wartshmicht was killed in single combat with the Vile Ork warboss before the xenos scum were driven back in the battle of Parisiana by the quick actions of Company Champion Sun Laotzu. The fall of Chaplain Wartshmicht was the beginning of the end for the warriors of the Fourth. Without his spiritual guidance, Captain Ferranus was easily frustrated by the next two years of battle against the Orks. He began to be swayed by the Chaos cults he was forced to use as foot troops, and so when he declared his secession from the Imperium of man, the fourth went with him to a man. Epistolary Botrejadre had already been pushing his powers well beyond anything he'd ever done attacking the vile Ork wyrdboys. He began experimenting and getting more and more reckless in his rites of purity in the name of survival. At the battle of Lamedon, Company Champion Laotzu toppled the massive warboss Grugnir SpineEater, the Ork's great champion, with one great swing of his mighty hammer, Foe Bane. This was the turning point in the battle. In honor of his great valor, Captain Ferranous had the Chaplain's revered suit of Terminator armor cleaned out and gifted it to Laotzu. During the civil war, after nearly all the Fourth's war machines were damaged beyond even Brother Smythton's prodigious skill to repair, the Techmarine along with Apothecary Renauld and six other Battle Brothers, simply disappeared. What became of them remains a mystery.