The planet Cepheus, located far in the east of the Ultima Segmentum had been refound by the Sisters of Battle several decades after the Horus Heresy. The Thousand Sons that had occupied the planet had deserted it years before in favor of pursuing their legion into the Eye of Terror, the delay being in part caused by a warpstorm that lingered light-years away that served as a nuisance and constant risk to the lone planet. The storm seemed to probe the communications of the planet, causing communication blackouts for years. These years proved to be most precious, preventing the Thousand Sons on the planet from retreating with their battle brothers. They were not spared, however, from mutation and the Rubric of Ahriman. After their confused retreat their movements revealed the location of the planet to the Imperium, which hastily captured and purged it of any heresy the Thousand Sons might have left behind. Sisters of Battle lead the purge and established a force to watch over the planet alongside the Imperial Guard.
A Tyranid Hive Fleet however has arrived to Cepheus at full strength unannounced and unforeseen. Prayers to the Emperor were raised from shelters as the Imperial Guard and Sisters of Battle clashed with the hungry swarm. A strange presence had prevented the derailment of the population by the behind the scenes manipulation of a Genestealer Patriarch. A surmountable defense quickly was organized against the Hive Fleet as a result. However through sheer numbers the Hive Fleet was sure to dominate. The psychic presence however was unwavering and had already been accepted by the population to be the watchful eye of the Emperor decades before the horrific Hive Fleet's arrival. An Adeptus Mechanicus cult on the planet understood it to be caused by a well hidden and cherished relic on the planet which they believed remained on the planet by the will of the Emperor and Machine God. A small group of the population itself worshipped the relic alongside the Mechanicus members and took great lengths to hide it from the righteously zealous Sisters of Battle after advisement by the tech priests.
Now this radical faith was put to the test as a handful of Sisters of Battle, Imperial Guardsmen, and civilians barricaded themselves within a gothic chapel where the relic resided.
"MAKE ME SOME MORE BARRIERS. THERE'S ONLY ONE WAY INTO HERE AND THAT'S THROUGH THAT DOOR!" An Imperial Guard captain yelled over the confusion and panic within the chapel. The few Sisters of Battle and Imperial Guardsmen worked ceaselessly to ensure the large door would hold back the next wave of Tyranids that would surely overwhelm them otherwise. The Mechanicus cult members were absent from these events that preceded the soon to arrive battle. The captain was quite sure they were possibly preparing for combat themselves within the depths of their ancient chapel, praying to the Machine God for protection. He could care less as to their whereabouts. Half an hour later he was satisfied that the door would hold back any Tyranid genus that dare strike upon it. Firing positions were established on the windows to rain death upon the attackers. The siege would be vicious.
Captain Nathaniel Warren might not have pursued to excel in the Imperial Guard so well if he knew of the Tyranid swarm that would arrive to tear him and his men apart. He was a simple man. Not much differentiated him from his men. However his capacity for leadership was exceptional after multiple engagements with brutal Orks. After retaking Cepheus his forces remained there to ensure it would not fall to a Xeno assault. Unfortunately this would be the only and last of his missions that he would ever fail.
"We might even hold them back long enough for the Mechanicus Cult to finish their hymns and praises." Amadis, a Sister of Battle, grimly noted. Captain Nathaniel ignored her and focused on the prospect of giving the Tyranids a hard fought battle for the chapel regardless of the inevitable outcome.
Amadis was raised on Cepheus under the teachings of the Adepta Sororitas and joined them with righteous zeal imbued in her heart. She had desired nothing more than to serve the Emperor. Though to her disappointment her potential to do so is about to be cut short by vicious evisceration by the claws and hunger of the Tyranids. Regardless she stayed strong, with the same resolve Captain Nathaniel held against the Xenos.
"Oh my- look at the size of it!" An Imperial Guardsmen shouted in horror. It seemed something was overlooked in the hasty construction of the barricade. A large Carnifex rammed the chapel doors. The chapel shook to the roar of the furious beast. Nathaniel Warren had an excellent plan in the making when he chose to barricade his men in the ancient structure, but he hadn't expected a Carnifex to appear so suddenly. Imperial Guardsmen and Sisters of Battle took up firing positions and fired upon the beast in an attempt to deter it from attacking the door. Their attempts at repelling was ineffectual however, as the beast continued to ram the door as though urged on by instinct. Hormagaunts swarmed around the monster's feet and were joined by Termigaunts which quickly moved to suppress the defenders by rapidly firing on the windows. The defenders were entombed. A sort of holy chorus erupted into the halls of the chapel, causing an explosion of confusion among the Sisters of Battle and Imperial Guardsmen. The Mechanicus Cult hailed the Machine God and the Omnissiah for deliverance from the grip of the Xenos and also raised their praises to one they described as a knight and weapon of the Emperor. The door cracked open and the Carnifex shoved its head in to get a better look at those that were about to be slaughtered by the waiting swarm of gaunts. The morale among the Imperial Guardsmen and civilians immediately shattered as death looked them in the eyes. The thunder of heavy metal on pavement coming from a chamber of the chapel caught the attention of those present.
"Nero joins you to purge the Xenos infestation." growled a deeply electronically synthesized voice. The roar of an assault cannon spewed forth the rounds necessary to put a crack in the Carnifex's skull, forcing it to withdraw its head in howling agony. A Dreadnought fully emerged from a dark passage leading into the depths of chapel. It was an ever loyal Space Marine entombed within a life sustaining sarcophagus of war, though what made this machine special was the fact it was a traitor legion Dreadnought. The Thousand Sons, distracted by their mutations, had unintentionally left a dormant pre-heresy hero behind that had not been swayed by Chaos due to their hasty retreat to the Eye of Terror. And so it remained behind waiting and watching the planet that it was abandoned upon. Its colors gave it the appearance of seeking repentance for the unfortunate turn of its traitorous legion. The red and white painted Space Marine Dreadnought marched forth with the Mechanicus cult members following in an orderly fashion behind the revered machine. They feared not any accusation of heresy for harboring the machine. Before the Sisters of Battle could consider turning their bolters on the Dreadnought and its followers the Carnifex returned with fury. It thrust in its claws and pried the door open viciously; effectively nullifying the barricade that would have held it in place should the Carnifex of kept ramming it. The Dreadnought quickly moved forward to engage the beast. Imperial Guardsmen and Sisters of Battle stood aside as the ancient machine rushed down the great chapel hall to the compromised doors. Realizing the barricade would not be there for long the Imperial Guardsmen and Sisters of Battle near the door took the opportunity to fire into the mob of gaunts that waited outside the chapel as the Carnifex and Nero charged the barricade themselves.
The Carnifex crashed through the barricade first. Imperial Guardsmen were immediately forced to fall back as pieces of the barricade were thrown about. However they valiantly laid down suppressive fire on the advancing horde of Tyranids. The Dreadnought fired its assault cannon as the space between the Carnifex and the Dreadnought dwindled to mere feet. Nero used the momentum of his advance to his advantage and threw a powerful uppercut with his power fist. Immediately the massive Carnifex was toppled into a wall of the chapel and onto defending Imperial Guardsmen. At this point the Sisters of Battle and Imperial Guardsmen retreated to the end of the chapel hall to avoid the confrontation but continue to defend. The civilians had already taken shelter within the depths of the chapel that the cult members had emerged from. The Carnifex would have recovered from the blow if it wasn't for the Dreadnought driving his mighty fist into the Carnifex's skull unopposed. The Dreadnought turned from the corpse and confronted the oncoming horde with the assault cannon. Already Hormagaunts were swarming over the Dreadnought, prying for weaknesses in the armor. Nero stared out from within his sarcophagus without fear. A weakness in the armor would not be found so easily as Imperial Guardsmen let loose volleys of lasgun fire to dislodge the vicious creatures. The shots that missed caused little wear on the ancient machine's renewed appearance but it was of no consequence. Nero was free to let loose his powerful weapon upon the chapel's choke point, the open door.
Nathaniel was dumbstruck by the Dreadnought's appearance. It bolstered the morale among his men to see a veteran Space Marine fighting alongside them. He was unaware of the significance of its colors however and welcomed the help without question. The Dreadnought might as well of been a gift from the Emperor himself sent forth for this plight that besieged his people.
Amadis on the other hand was confused and zealously enraged. She couldn't decide what to make of the appearance of a traitor legion Dreadnought coming to their aid. It was still adorned with the Thousand Sons' pre-Heresy colors but she doubted that its heart had escaped the taint of Chaos. Nero would have to be eliminated. Not at this moment of course, even she could realize that destroying the machine now would be to their disadvantage. It would have to be dealt with later after the Tyranids were dealt with…if they could be dealt with.