Excerpt from the historical archives:
785.M41 - Chapter Master Claudius of the Doombringers led his veterans and 4 companies of marines against a heretical cult that summoned a greater demon in the Porrtag Tral system. Claudius fell in battle and was succeeded as chapter master by Brother Captain Vespasian.
The true story is not nearly so clean.
While the bulk of the Doombringer force was bringing the capital city under control, Chapter Master Claudius led his company captains to the temple at the source of the incursion. The five disembarked from their transport and were met by a dozen twisted aberrations surrounding the temple. Vespasian, captain of the vanguard, jumped into the fray, scattering creatures with his spear. Captain Otho doused several in promethium, while Master Claudius put down the stragglers with a pair of well placed bolts. Vespasian jetted up to the guardhouse atop the outer wall and released the lock and shield on the gate. A twisted fusion of demon and armored walker broke through the unshielded stone doors only to meet a hissing death from Vitellius’ multimelta.
Inside the courtyard, they were met with horrors they had seen too many times. Rotten and disfigured bodies covered in the symbols of chaos were strung up on the walls. Even though they were obviously dead, their mouths moved, whispering vile secrets just below the threshold of hearing. From there they moved to the ruined opening that used to be the main doors of the chapel. All that could be heard from outside was the inhuman chanting of a chorus, reaching a fever pitch.
As they stepped inside the chorus quieted and they witnessed a statue of the Emperor taller than any two of them sink into a pool of inky blackness that absorbed all light. The captains dispatched the chorus of cultists, but it was too late. Whatever their purpose had been here, it was accomplished. The dark pool rose and swelled to form a sphere. The captains sought cover and readied their weapons for anything they thought would come from it. They couldn’t have anticipated what would happen next.
The sphere exploded, extinguishing all light, all sense of direction. Form and force were lost in a swirling soundless maelstrom. Vespasian found himself floating in a void. But he was not alone. He felt a presence, and felt rather than saw a great blue reptilian eye, examining him the way a scientist might regard a cell through a microscope. The utter apathy and power of the presence pressed at Vespasian’s sanity til he felt his head might burst. When he thought he could bear no more, it ended. The presence had passed judgment upon him and he felt heat return to the world.
Opening his eyes, Vespasian saw the interior of the temple as if nothing had happened. Until he turned around. His brothers writhed on the floor and their armor pulsed as if alive. A voice spoke from within his helmet, “You are now mine. They shall be… a gift to my brothers.” And his former brother-captains rose from the floor, Changed. Their crimson armor was now a lurid green and each bore hideous mutations.
Vitellius, once the aged captain of the devastators, had swelled with rot and gaping wounds and the weapons that had once been his tools now seemed to grow at random out of his arms and chest. Galba, who had led the sternguard with distinction for a century, at first looked untouched. Until you saw the tendrils that now wrapped his gun in place of hands. Even the mighty bolter itself was changed into a gruesome thing that sang with an unearthly melody. Otho, always the first to the front, leading his brothers bravely in the thickest of the fighting, grew spikes through his pauldrons, blood flowing profusely through the holes. No trace of Claudius could be seen.
A terrifying fight ensued. Vespasian vanquished his warped brothers, but even once they were dead, the torment wasn’t over. The voice returned, coming from his left ear, “Excellent work! I could not directly eliminate those daemons. But I could direct them to those bodies. Now for your reward. HA HA HA HA HA HA…” The voice echoed, growing in intensity as it worked its way around the temple. Vespasian saw the blood on his hands boil and his armor start to turn green. Strangely enough he could only see this through his left eye. Before the change was complete, Vespasian tore his helmet from his head, cut off his left ear and put out his own left eye with his spear.
The changes were gone and the voice stopped, but a whispering wind kicked up in its place. It heightened to a roar and then the blood soaking the floors ignited. It burned higher and hotter than any fluid had a right to burn. Vespasian got out of the temple just in time. The inferno engulfed the chapel and spread to the courtyard. Vespasian got to the transport a moment before the flames.
In orbit, he heard similar stories emerging from the battle brothers that escaped the planet. Tales of green armor, brother fighting brother, and flames. The Doombringer’s strike cruiser’s sensors showed the entire world was ablaze. Under Vespasian’s orders, the strike cruiser loosed the remaining torpedoes against the capital city and the temple and broke orbit as the conflagration spread to space.
In the aftermath, the veterans refused to abandon their chapter in its dark hour. Those that were still able were interred in the chapter’s dreadnoughts.
The Doombringers have survived increased scrutiny from the Ordo Hereticus throughout the last few centuries primarily due to the utter lack of evidence. The chapter’s losses have been ascribed to the battle and the survivors have proved free of taint during investigations. With rumors of chaos ascending in Porrtag Tral and the temple restored, the Doombringers are returning to finish the fight started in that temple so many years ago. And perhaps keep any remnants of their close brush with chaos away from the Inquisition.
The Doombringers then return to the Porrtag Tral system. It's been occupied by the Tau and a combined IG Blood Angels fleet attempt to break the blockade, only to get thrashed by Eldar interference. Silly elfs.
The Doombringers arrived too late to assist in the initial orbital assault, and by then the xenos were nowhere to be found. Chapter Master Vespasian sent several scouting groups down to the planet under the leadership of the chapter’s librarians to the region where the temple had stood.
Librarian Codicier Vryce discovered the temple and set up a defensive perimeter. The Doombringers marines set up heavy weapons teams in the nearby towers and built protective bunkers and laser fences around their mobile elements to channel any enemies into a killing zone.
Vryce radioed the discovery to the Doombringers’ command cruiser. A squad of terminators in close assault gear teleported in to join Vryce in securing the interior of the temple. Little remained inside the ancient walls. Broken stones littered the courtyard, long since washed of the blood and soot. Inside the chapel, there was no evidence of any past heresies, but a more thorough search revealed a secret entrance to catacombs many thousands of years older than the temple above. Codicier Vryce and the terminators descended into the dark corridors but as the last terminator passed through the opening, it collapsed.
On the surface, a series of violent explosions of purple energy ripped through the complex. Ghostly green marines materialized outside the walls of the temple. Their bolters fired eerie blasts that tore through the Doombringers’ armor without slowing and most of the Doombringers’ own rounds passed through the ghosts harmlessly. All the dreadnoughts, speeders and razorbacks from the other scout groups raced to join the battle.
Vryce heard the concussions above and opened his mind to the warp to sense the twisting tunnels around him. He felt the presence of a dark and powerful being pull his attention to a particularly deep section of the catacombs, but he ripped his focus away and found a path to the surface. He broke into a run, the terminators close on his heels.
Vryce and the assault team broke out of the thick brush covering a nearby rockface into the middle of the firefight. A winged figure strode out from the middle of a crowd of ghostly marines. The translucent figure spoke something unintelligible, and Vryce felt him reach out at him through the warp with a corrupting tendril. Vryce poured his will through his psychic hood to drain the energy from the tendril.
The terminators’ lightning claws tore through the ghostly marines surrounding the mysterious sorcerer, but as the thunderhammers swung down, the sorcerer dissipated into a green cloud with a shriek. The rest of the ghostly marines disappeared with him. Vryce reported the incident to the Doombringers’ command cruiser and called for the chapter’s apothecaries to tend to the dead and wounded. On a private coms channel, he conferenced with Chapter Master Vespasian and Chief Librarian Telemaius.
They would have to deal with the sorcerer and the dark presence that had been awoken.