Originally, this was intended to only be a very short extract of text, but seeing as it got favourable reviews on the site I subjected to it, I'm planning to extend it to a multi-part story. If it recieves good reviews here, perhaps I'll create a second (and much longer) chapter. Any and all criticism welcome, even if it's to tell me I am to be executed for heresy.

It started with the slightest of tremors.
Were an astropath to monitor the Warp route to Calix Beta at that moment, that was how they would have described the activity in the Empyrean. By the standards of the Warp, it was akin to a calm sea; perhaps disturbed by the daily movements of minor sea life. There was certainly no indications of anything suspicious.
Galen smiled, and gently squeezed the arm-rests of his throne. His claws dug into the obsidian, and scraped along it's ornately carved surface. Before him, the bridge of the Pius Cultus hummed gently, though not a single movement could be seen. Galen's grip tightened as he recalled the days when the bridge had buzzed with activity, when the crew of this once-gallant vessel had worked with righteous passion to bring justice to the Emperor's enemies.
Galen let slip a chuckle, the hiss of his breath reminiscent of a furnace. In truth, it wasn't breath that escaped his lips; he had ceased to breathe a long time ago. Galen laughed louder, enjoying the irony of his ship's name. Times had changed. The bridge was now empty, the ship infested with entities similar to himself. It was they who ran the vessel now. The warriors who had guarded the Pius Cultus, too, had long since ceased to be the Emperor's noble Adeptus Astartes. Galen himself had seen to that.

As his mind began to wander, Galen thought back to those times.
It had been different, once, back in the glory days. Thaddeus Galen, the celebrated Chapter Master of the Sable Knights, had once been a mortal, a hero of the Imperium's armed forces. His was the hand that had laid low the infamous Khargun the Unspeakable, pirate lord of a host of traitor Astartes. He had led his chapter personally against the forces of the Despoiler during the 10th Black Crusade, in the years preceding his own fall. His chapter had once been spoken of with pride and near-worship as paragons of Imperial Virtue.
It had been the Inquisition's appearance on Syracuse, their home planet, that had presaged the doom of the Sable Knights. Over the centuries, Galen had authorised his librarians to gather as much lore on the Great Enemy as possible. After all, was it not wise to know your enemy? Still, the collection of supposedly "forbidden" tomes had aroused the suspicions of one Inquisitor Raav. Galen remembered that man; the arrogance of the wretch, storming into the Sable Knight's fortress-monastery, and the accusatory tone of his voice as he levelled charges of Heresy and corruption at the Chapter Master. Galen had reacted with fury, and demanded to know what the Inquisitor meant by making such accusations. The Inquisitor had pointed to records of hundreds of forbidden texts said to lie within the grasp of the Chapter Master.
Galen fumed at the nerve of this whelp. Did he not understand?
Ultimately, though, Raav was right. As he recalled those dark times, Galen felt the dead weight of grief settle in his chest, displacing the earlier annoyance. Raav had been correct to accuse him of corruption; he had spent too much time reading those books, poring over the knowledge they contained. What he had learned, what it had cost him to do so... it horrified him. He knew what happened to souls in the warp, what would happen to his own when he died. The knowledge sheared his mind like a knife; he felt an icy terror grip him, undoing all of his years of Astartes training and combat experience. It cut him to the core.
For many nights, Galen had not left the sanctity of the Librarium, desperately hunting through his illicit collection to find a way to avoid such an awful fate. On the fifth day, he found something; a reference to an ancient codex of daemonic power, the Eternus Specialis. Galen knew he must have it. The legends of the book took his chapter to the depths of the Eastern Front, to a dead world. There, the hidden temple of the book was to be found. There, Galen would have his answer.
His warriors took to the surface, each making his supplication to the Emperor and Guilliman. Galen waited until nobody was watching before offering a prayer of strength and fortitude, gently placing a seal of protection to his breastplate. Then they had advanced into the temple together, a rough-hewn thing cut into the side of a deep valley.
They found what they were looking for.
As the Astartes reached the centre of the unholy place, the air seemed to shimmer. The warriors raised their guns as the light flickered and danced. Galen remembered the next few minutes with acute clarity. The battle-brother next to him had been lifted up into the air, impaled on the blade of a materialising fiend. Other horrendous creatures, their skins slick with gory pulsating muscle and their eyes ablaze with hellfire, attacked, their inhuman howls in stark contrast to the roar of the Sable Knight's boltguns. Galen had drawn his blade and hewn a daemon-thing in two before he realised the situation was hopeless. If they stayed in this place, they would all surely die. So he ordered them to fall back, holding back the urge to run. The warriors he commanded must not know of his fear.
They had made it in the end, Galen recalled. Scarcely ten of the twenty brothers who had accompanied him were on the Thunderhawk out of there. Still, it did not matter to Galen. He had his prize.
Three months later, the Inquisition returned. His theft of the book had been noted, it seemed, by that insufferable meddler Raav. Worse, he had declared Galen excommunicate. When he heard, the Chapter Master's hearts nearly stopped. To be declared excommunicate meant he was fair game for the vengeful Inquisition. With his life at stake, and his chapter refusing to answer the call to surrender, speeding the war to come, Galen had made his final, desperate plea to the Emperor to spare his life. The Emperor did not answer him. Something else, on the other hand, did. Galen smiled as he remembered the voice. A kindly, fatherly voice; one filled with concern and sympathy for his plight. No cold, distant deity, but a real, tangible being. The Voice had soothed him, and offered a solution; the voice needed souls, plenty of them, and for such a gift it could grant Galen not only immortality, but the means to strike back against the Inquisition that had so blighted him without fear of reprisal. Galen took one look at the book in front of him, and accepted.

Galen's recollections ended abruptly, as the hull of the Pius Cultus shook and groaned. The tremor of warp activity became a massive wave, then went silent. He was back in the material realm, and had no need to hide his ship's presence any longer with his psychic will. Galen snarled, and stood, his massive form shadowing the possessed bridge with his magnificent bulk. He made his way down to the launch bays of his beloved Battle-Barge, and there, he found his army awaiting him.
Nurgle had not lied when he had promised Galen an immortal army. There they stood; the Black Host, formerly the Sable Knights; loyal Astartes no longer, each warrior was a soulless slave bound to Galen's will. Their bodies, rotten and mutated to better kill in Galen's name, would not age or die. Concepts such as mercy or restraint did not bother them; they would kill and kill until ordered to stop. Though they acted with the same skill and lethality they had done in ages past, Galen knew the truth. With his immortal army behind him, and Father Nurgle watching him, the Daemon Prince was no longer afraid.
But the citizens of Calix Beta soon would be. Galen had tracked his former persecutor Raav to this planet, and as he prepared to hunt him down, he could not suppress a grin. It spread across his hideous visage, revealing his venom-riddled fangs.
Oh, yes. Raav would learn the meaning of terror.

Five hours later

The skies were burning.

Alvaros Raav stared up at the conflagration. Gently, he turned away from the window, and faced the room's other occupants.

"Gentlemen, the situation is grave."

It had started two days ago; the first sign of the storm to come. The system patrol ships of Calix Beta, en route to what they assumed to be a minor pirate entering the system, had disappeared. Even now, Raav was unsure whether they had ever been confirmed destroyed. They just seem to have vanished; not a trace of them was to be found.

Then the sky had began to burn, violets and blues and reds all mingling into the unnatural flames. Under any other circumstances, it might be considered beautiful. Of course, these were not any other circumstances. From what little information they had gleaned, it appeared that an invasion of Calix Beta was underway. Raav silently watched, appraising each individual as Field Marshal Larrens, overall commander of the armed forces on the planet, continued his briefing.

"Due to apparent inability of the system ships to contact us, we can only assume they have been destroyed during their mission to investigate the anomaly in the outer system. Three hours after their last report, a ship of unknown designation attacked and destroyed the planet's only listening station. Whatever this vessel is, it must be very well armed and armoured, as that station was no lightweight, and despite this it was crushed before we could get a comprehensive report from it. What we do know, from the information we did receive, is that it easily matches our capital ships in terms of mass, and that it wasn't alone; two more ships of roughly the same class were registered as entering the system by the station before it's destruction. Furthermore, just before the station went offline, our astrotelepaths reported psychic screams and howls from the vicinity of the battle, such as they had never heard before."

Larrens paused, gently flexing the fingers on his mechanical arm, before continuing.

"I think it's pretty clear, given the evidence, that we aren't dealing with pirates or some rag-tag rebels. That enemy ship struck quickly and precisely, and hit us where we were most vulnerable. Clearly whoever is in charge of this force knows their business. No, gentlemen, it is my belief that we are looking at an invasion of the system."

Raav nodded in approval of the blunt tone of the statement, and returned to his examination.

In front of him sat every single powerful individual on Calix Beta, each a master of his or her own domain on the planet. Adept Lertaius, representative of the Administratum, Magos Hyferian, master of the Calix manufactorums, Chief Judge Donnegan, leader of the Adeptus Arbites, Governor Kallas, Imperial Commander of the entire sector; they were all here, as well as a select few of their chosen inferiors. Raav had introduced himself personally to only a handful of them, and had made sure to keep aloof from the ones he distrusted. Though some of the gathered worthies pleased him with their quality, Lertaius worried him. The man gave off the attitude of a hidebound bureaucrat, something the seasoned Inquisitor was worried would become greatly frustrating when the realities of war began to bite. A reed that did not bend in the wind would break; Raav feared Lertaius to be that reed.

Though, Raav thought snidely to himself, you too were as loyal to the rulebooks, once.

Larren had finished his initial speech, and was now turning to the Inquisitor. Raav liked the man; his scars and bionic limbs showed that Larren was no coward, and had fighting spirit. He would need all of it if Raav's suspicions were correct. The inquisitor had no reason to doubt them; the last great mistake he had made had been over a century and a half ago.

"Of course, as all of you know, when a threat rears it's head, the Imperium responds accordingly. As such, Inquisitor Raav has appeared at just the right moment."

The reaction of the room was varied, but dramatic.

Lertaius gasped, and went pale. Hyferian looked up sharply and examined Raav with his beetle-like biomechanical eyes, Donnegan's face seemed to harden and become as stone, and Governor Kallas frowned, his brows almost meeting and his eyes clouding with worry. Raav had seen it before; most people did something like this when they learnt of his role within the Emperor's realm.

"Perhaps you can shed some light on the situation, Inquisitor?" Larren inquired.

Raav nodded to him, unwilling to speak at first, and moved onto the speaker's podium, allowing Larren to step down first.

"I am afraid I do, though the news may come as very unpleasant."

Raav faced the room's occupants and fixed them all with a stern look.

"This world is under attack by no ordinary foe. The sky, as you will have noticed, is burning; I have seen such dire portents before. You were wiser than you know to mobilise your regiments of PDF and Imperial Guard as fast as you did; you will need all of their fighting strength and more, I think, in the weeks to come."

Raav gripped the pulpit in front of him.

"Prepare yourselves for the hardest war of your careers. Chaos has come to Calix Beta."

The room was silent, so completely so that Raav could actually make out the humming of his own powered armour.

Yes, they believed him. That was good; it meant that they would co-operate much more readily from the outset.

"I have fought and prevailed against the Great Enemy before, and I intend to do so again. I am aware of this world's military reputation, and the various attacks it has repulsed. It will need to live up to such an honourable record if it is to survive."

Kallas spoke, his voice recovered at last. Raav felt surer of his decision to be so blunt when he heard the governor's words.

"Have we attempted to make contact with the rest of the Imperium? If the threat is as great as you say, Inquisitor, it would be no small comfort to know we did not fight without hope of reinforcements."

Larren nodded. "I had it done before the meeting began."

"Good." Raav nodded in approval. "Now, listen to me. The last known contact we had with the enemy vessel was three hours ago, or thereabouts. By now, it will have almost reached the planet. If our defences are not ready by now, they must be in the next twenty minutes. Our best chance rests on the enemy not gaining a solid foothold on the planet, for if they acquire such a beachhead, they will be able to deploy in full. That, given the nature of our enemy, is not a desirable situation."

Larren and Donnegan nodded, slight approval playing across the worry written on their features.

Raav drew himself up, a parting in his cloak revealing the powered armour underneath.

"Under no circumstances must Calix Beta fall. We cannot allow this blight to escape and plague nearby sectors; they must be held here."