The Seraph

In the valley below the mountain peak, the Emperor surveyed the aftermath of the battle that had defeated the Garrison. His Custodians and the Astartes of the Imperial Fists Legion had been victorious and Ophelia had been pacified. Ever the strategist, seeing this system's value to the Imperium, Rogal Down began to speak about ferrying colonists. But even as he spoke the Emperor interrupted him.
"Take charge, Rogal," he ordered. "Verity, attend me." Without another word, he turned and boarded one of the Custodians gravtanks, accompanied by the puzzled Tech-priestess and a small cadre of his bodyguards.
The Emperor had sensed the Warlock's death, hearing his psychic scream. Dire portents swirled in his precognitive vision - he knew the fate of this world, perhaps the galaxy, hung on whatever might happen in the next few minutes. Goading the driver of the tank to foolhardy speeds up the mountain, he leapt from the vehicle even before it landed, Verity jumping out beside him and his Custodians following an instant later.
He did not waste time asking Sigismund what was happening - with brutal efficiency, he sucked the immediate knowledge of the situation from Sigismund's mind, leaving the First Captain slumped mindlessly in the dirt. For Verity's part, she approached her brother magi and - through the neurotrans connection they shared as members of the Mechanicum - sensed what they were doing.
"It's a lie!" she screamed. Her unique abilities and devotion to the truth allowed her to see what the other Tech-priests had not; that whatever was imprisoned inside the Crystal Vault had deceived them into setting it free. "Don't open it!" The Tech-priests ignored her, alien fanaticism gleaming in their eyes, and kept unlatching the locks. "Stop them!" she screamed.
Trusting her - as one of his oldest allies who had not only never lied to him, but was incapable of it - the Emperor sent a psychic command and, as if they were his own limbs, the Custodians moved forward, halberds raised. But they were too late. The crystal sphere opened into a gateway to labyrinthine maze of crystal corridors and an ancient, cybernetic horror burst forth - a six-winged serpent of azure crystal and alien metal. The Vault was open and the Seraph was free.
The Tech-priests were absorbed in a twinkling, what little organic flesh remained splattered across the rocks and their bionic enhancements incorporated into the Seraph's body. A dreadful techno-psychic scream echoed from the mountaintop as the Custodians closed ranks around the Emperor and the Marines opened fire. Their weapons blew chunks from its body, but it reached out with its newly-acquired mechadendrites and plugged itself into their armor, overloading the suits' programming and circuits with ancient, alien code. It took mere moments - the Marines shuddered inside their armor as biological recycling and recirculating systems worked against them, starving them of oxygen, flooding their blood with toxins and frying their nervous systems. Moving jerkily but with dreadful speed, the suits turned their guns on the Custodians.
Verity understood what was happening - her brief contact with the alien programming had revealed the truth. The Seraph and the Vault were the same thing; an ancient, alien intelligence converted to mechanized form to fight in a war older than the stars, defeated but not destroyed, reprogrammed into crystal quiescence and then buried within the mountain. The Vault was the Seraph's body, forced to assume that shape by the imprisoning programming. The six-winged serpent fighting the Custodians was merely a fragment of a much larger horror, the small part of the monstrous weapon-creature the Tech-priests had unlocked. If the locking program was not reinstated, the Seraph would free the rest of its coils and kill every Imperial soul on the planet, and perhaps beyond.
Verity knew what needed to be done and as she looked over at the Emperor so did he. Silently, he nodded with solemn sadness at the daughter of the Mechanicum who had been one of the first to acknowledge him as the Master of Mankind. She flung herself at the Seraph, her own mechadendrites interfacing with it. Alien programming invaded her mind as she and the ancient monster connected. Desperately, she dove into its awareness, its very mind, feeling the weight of eons and a mechanical hatred for all flesh flood her. She felt her own blood and bone slough away from her cybernetic enhancements as it absorbed her body, but not before a golden warmth enveloped her. Where the other tech-priests had been reduced to nothing more than soulless circuitry which the Seraph's programming had ruthlessly dominated when their organic minds were destroyed, Verity felt her awareness expand as the Emperor himself flooded her soul with a modicum of his inestimable power. Verity - a hybrid fusion of the machine and the human - plunged deep within the coils of the Seraph's programming and fought it with all the fervor of humanity as the Custodians charged its stumbling form and shoved it bodily back into the crystal labyrinth, the gate closing behind it.
Inside the maze, the Seraph's serpentine body writhed with internal conflict. The sophistication and power of its ancient programming was greater than anything the Mechanicum could muster, let alone the cybernetic augments of a lowly tech-priestess. But it was not crude logic circuits that fought the Seraph, but the flaming spirit of Man itself. Against a human soul, illumined by the Master of Mankind himself, no mere machine - even an ancient alien intelligence millions of years old - could hope to prevail.
But Verity's victory was neither instant nor without cost. Her body she had already lost, its organic components sloughed off and splattered on the mountainside. Now as she struggled against the malign intelligence of the Seraph her mind and soul merged with it, the two of them fusing inextricably together. As she struggled to retain her identity, she warred for control of its - her - body. When she was dominant, she retreated further into the labyrinth. When the silica animus was in control, it attacked the Custodians, crushing them like insects. Soon, only the Seraph - its body a fusion of alien crystal and Mechanicum bionics, its mind a war between human soul and alien programming - remained.
Outside, the Emperor was alone except for the grav tank's Custodian driver and the drooling Sigismund. The First Captain was stunned, but his awareness was returning. Swiftly, the Emperor used his formidable psychic power to summon a thermokinetic wave, melting and reforming the stone to cover the entrance to the Crystal Vault. He turned to see the driver standing over Sigismund, his halberd poised to execute the Imperial Fist. The Emperor shook his head and, with the merest expenditure of his mental energies, wiped the whole event from the First Captain's memories. He did not even need to speak to the Custodian. "This Talon is sworn to secrecy, Sire," he assured the Emperor. "This Talon shall not even think of it." Nodding in satisfaction, the Emperor picked up Sigismund and boarded the grav tank, ordering the driver to take him to Rogal Dorn.
Within the mountain, Verity was doubly trapped - within the maze of the Vault, and also within the Seraph's body. Her soul, the indomitable human spirit, was victorious over the machine, but she knew her fight was only just beginning. Only part of the Seraph was free, the smallest portion of its programming unlocked. It was that she had fused with, but the rest remained beyond her reach, imprisoned within itself by encryption code locks. It was securely quiescent for now, but she could not be certain others might not dig into the mountain and, like her tech-priest brothers, fall prey to the Seraph's seductive lies. Immortal now she was joined to the xenos programming, victorious at the cost of fragments of her soul and with pieces of its own identity embedded within her, she deliberately unlocked the next segment of the Seraph's programming and plunged into combat. If it took her ten-thousand years and everything within her that was human, she would defeat this monster and take control of everything it was. Guided by her unshakeable faith in the Omnissiah, she began her lonely battle.
In the valley, the Emperor spoke quietly with his Primarch, telling him the barest minimum necessary. All detailed records of the campaign were to be destroyed - it would be recorded as Victoria Imperalis but nothing more. The Ophelia system would be colonized, but an eternal edict forbidding excavation or construction within the mountains of Ophelia was to be set down. The rearguard Marines who had fought the Eldar were brought to him and he interrogated them personally, scrubbing their minds down to those of newborns to divine every single nuance from what they had seen and heard. As the imbecile Astartes gurgled and drooled in front of him, he sat in silent contemplation of what he had learned and certain knowledge of the threat that would come when he was no longer there to protect humanity.
Finally, despite the fact he had taken every precaution to hide the truth, the Emperor ordered a monument to Verity be made. Perhaps it was a purely human weakness, an acknowledgement of the sacrifice she had made. Disdaining and abhorring the Mechanicum's use of bionics, and knowing it was the spark of humanity which would allow her to defeat the Seraph, he ordered his genitors to project her unaugmented appearance in the prime of life through the use of DNA prognosticators and sculpted an allegorical representation of her defeating the Seraph from this pattern with his own hands. He set the statue atop the mountain with the legend "VERITY VINCIT VERMIS" on a plinth beside it; "Truth Conquers the Worm" in High Gothic.
Encircling the monument to Verity, he erected ten pillars in the Palatine-Terran style to honor the Custodians who had sacrificed their lives. This duty done, he walked down the mountain alone, through the valley where he had defeated the Garrison. He met Rogal Dorn and his court at its mouth, the leader of the colonization fleet with them. He brushed aside the Primarch's introduction. "You know my edict, Governor?" Nervously, the man nodded. "Then see it is followed." He did not wait for a response, but simply turned and boarded the shuttle to take him back to the battleships of the First Exploratory Fleet.
No other human would set foot in the mountains for nearly three-hundred years.