AN: Decided that a Codex Entry type deal was more fitting for Darkness, and I kind of just wanted to try it out. Someone wanted a Darkness!Serras Interlude. Well, here it is. Did this entirely for fun.
A three-way war unfolded, both on the surface of humanity's homeworld, and in the space above it. On one side, the loyal Primarchs and Legions of the Imperium fought to halt the advance of the forces of Chaos. On the second side, traitor Primarchs and heretics of both the mundane human and Astartes flavors, willing, tricked, coerced, or simply possessed, fought to slay the closest thing to a god of Order and Anathema to their own gods.
Blocky, gargantuan ships waged war on each other in space, ships being shattered ever few minutes even as Mechanicus adepts frantically worked to repair the vessels that survived long enough to limp to the Stellar Bellows and it's shipyards to the point where they could fight again. The twisted hulks, literally bleeding from wounds inflicted by their more utilitarian loyalist counterparts, were stronger than the loyalist's in may ways, but lacked proper cohesion, even under Horus' direction, which left many openings in their ranks, which were taken advantage of with the ferocity of the desperate.
This desperation was not born merely of the proximity of the Primordial Annihilator to the Emperor, who many believed to be a god despite his insistence on not being worshiped, though that was certainly a strong motivation, but because of the stranger-still shapes and beasts that attacked both sides. Twisted parodies of ships, living things, and sometimes both. While many lacked actual armament, reduced to simply ramming themselves into the enemy, Daemons shied away from the abominations.
No matter how many times they were destroyed, they simply pulled themselves back together again. Psychic might slowed this down, but even the weakest would recover from utter obliteration in a matter of days. The strongest would regenerate from what should be lethal damage even if a Primarch personally slew them, often in a matter of minutes. If not slain by a psyker, the shadows would recover in seconds what a Daemon would need a century to restore themselves from.
The shadows were led by a cabal of Malice's followers. Most of the lesser members commanded Malice's actual Daemons, who had less potent regeneration but were capable of actual thought, as even they were less than at ease around their 'allies' of the negative photon reading persuasion. The vast, vast majority of the shadows, from the Battleship-sized one dueling a Chaotic and Imperial Battleship at the same time and gaining ground through simple virtue of the damage failing to stick, to the lowly Shadow, the size of the common dog and roughly as dangerous, was Serras Salnus. To be more exact, half of her.
During the Istaavan V Drop Site Massacre, Serras arrived late, a rogue planet dropped in her fleet's path by a powerful ritual boosted by corrupted Hydaelyn Crystals and a million psyker souls coming very close to destroying a large portion of her Legion, and the abrupt stop she forced her fleet to undergo damaged more than a few of the engines and Gellar Devices on realspace entry so close to a gravity field. The necessary repairs caused them to miss the first 2 hours of the conflict, a situation which only grew worse when Horus personally assaulted Serras, teleporting onboard via a hole in her defenses.
He had captured the soul of one of her lieutenants, and the presence of said soul, presented as if it were a key, made her drop her guard for a split second. With the power granted by Chaos, that moment was enough. Horus very nearly slew Serras with his opening strike, but he'd come prepared for that failing. He had a Techmarine under the purely mundane leash of her mortal family held hostage invert the Gellar Fields, sending the 21st Primarch tumbling into the deepest depths of the Warp.
While the exact sequence of events did not become clear until much later, the psykers of Serras' Legion had detected Horus teleporting onboard, and swiftly came to the conclusion he was responsible for the apparent murder, or at very least attempted murder, of their Primarch. Their rage blinded them, and nearly half the fleet pursued him recklessly, while the other half attempted to interdict Horus' forces in orbit.
After the battle, the Legion nearly fell into despair, but was swiftly cheered when their Primarch stepped out of the Warp as if nothing happened, the personnel on her ship in tow. When she spoke, however, it was immediately clear something was terribly wrong. Her inflections were as if a servitor was attempting to mimic human emotion, her usual warmth was feeble, like a dying flame, and there was an almost robotic quality to her little quirks of movement, as if she were unsure why she was doing any of it, only that it was something she was supposed to do. Were it not for the familiarity of her soul, accusations of deception would surely have drowned out anything she might have claimed. As it was, more than a few of the captains, who had always been closest to her, were visibly suspicious of their returned Primarch, but held their tongue until she finished.
While in the deepest depths of the Warp, Serras and her personal ship had been swarmed by Daemons at first, as one would expect, but the common Daemons of Chaos were swiftly replaced by those of Malice, and a few true shadows, born of his followers who'd succumbed to Darkness. Malice had torn the vessel in half, but managed to not harm the crew, which made Serras pause in confusion as to why he would bother just long enough to let him talk, seeing as the longer she had to think before they were swarmed by an infinite number of Warp-bound creatures the better, she let him speak. Her attempts to create Light Crystals sputtered and died before they began, the Darkness suffusing the Warp snuffing out the very concept of light with extreme prejudice, so her options were few to begin with.
Malice had informed her of the true nature of the shadows, the Heartless, as beings that were essentially involuntarily incarnated Daemons of the souls of those they were made from. He also informed her that Darkness was his ally, and the only way she was going to get out of an abyss this deep was with his, and by extension it's, help. Surrounded, with thousands of Astartes and crew members about to experience a fate worse than death if she didn't accept Malice's help, she reluctantly asked what his price was.
Malice simply asked that she accept his help. Absolutely certain that this was a trick, but seeing no other option that she and her followers would survive, she agreed. Unfortunately, that was close enough to accepting Darkness that her defenses against it were compromised. Darkness was like an animal. It didn't do 'sometimes' the way a sentient being did. You used it, or you didn't, and if you used it, it used you.
With Malice personally present, her very being was flooded with Darkness sufficient to snuff out a sun. She succumbed almost instantly, splitting into a Heartless that appeared much the same as herself, save that it was a few shades darker, and an entity made up of her body and the remains of her soul, which due to having the core of it's being removed, lacked the ability to feel emotions very strongly, for the moment at least, and was thus reduced to running off of her memories like a machine. It would take years for a full recovery, if one was even possible. Malice, seeing no use for her shell, and having already gotten what he wanted, let her leave without any further complications.
There were more pressing matters however. With the revelation that 2 supposedly loyal Legions and their Primarchs were in fact Traitors, the conflict between Chaos and the forces of Order stood nearly on a knife's edge. Meanwhile the Heartless crafted from her corrupted soul core was going to carve a path from wherever it was spat out, likely the Eye of Terror, through the worlds between the horde it led and Terra to gather... recruits for a terrible task. There was a method by which Darkness could corrupt the universe wholesale, but it required a world of a certain narrative weight, simply due to how Darkness and the Warp worked. Terra was the best choice for that purpose.
The universe began as a mass of potential. The exact trigger would likely never be known, but reality exploded into being from that mass, dividing the real from the unreal. The point where this occurred was known as the Well of Eternity. Should a sufficient mass of Darkness be injected into it, the corruption would spread like wildfire across reality, dragging everything into the abyss. Swallowing Terra would allow Malice to forge a path from the narrative 'heart' of the galaxy to the 'heart' of reality. While Chaos would merely reduce the Imperium to a twisted shadow of itself doomed to a slow degradation, Malice would doom reality as a whole to the end point of that degradation immediately were he victorious. As such, Chaos was to be ignored until Malice's forces were dealt with. Humanity would survive if Chaos won. Malice would not offer that mercy.
Needless to say, this decision, combined with how off their returned Primarch was in general, caused a great deal of controversy amongst her Legion. A full quarter outright refused to follow her, choosing instead to harass Chaos and aid the worlds beleaguered by their raids. This mostly consisted of quelling the rebellions being triggered by Chaos so the other Legions could focus on protecting Terra. Serras simply let them leave, seeing no benefit in forcing them to follow her when it would likely consume more manpower than she'd lose simply acquiescing and increase the resentment they felt, and lacking the time to convince them.
Serras' remaining followers confronted her dark half repeatedly, slowing it and the horde of Heartless down to avoid them reaching the Throneworld in a timely fashion. She did not think informing the general public that she'd had a major component of her soul ripped out and made into a Daemon would be helpful for them to know. Quite the opposite in fact. Thus she had informed Imperial forces that a dark doppleganger of herself had been constructed by a sect of Chaos entirely out for itself, though she informed the Primarchs of what it's true nature was.
This led to the current situation. Serras' Heartless led a tidal wave of shadows against Chaos and Imperial alike, while the dregs of the Silver Sentinels, badly depleted from constant conflict with a foe that combined the worst aspects of the Rangdan, Orks, and Daemons, and given little chance to replenish their numbers while constantly campaigning, served more as advisers who taught the other Legions what they knew of combating the shadows, though how much they listened varied wildly for the first few battles.
Serras' shadow and remnant were both destroyed after the remnant burned itself out in a massive conflagration of Light to permanently destroy the shadow, who had already been felled three times by Chaotic and Imperial Primarchs but kept returning moments later as if nothing had happened or she'd spontaneously become a Perpetual. It was shortly after this that Chaotic forces finally punched through the Imperial Palace's defenses, taking advantage of the holes torn in them by the now-fading shadows.
Horus struck a mortal blow on the Emperor, distracted by a final spiteful gambit by Malice to snuff out Sol, and was obliterated in turn, Sanguinis personally carrying him to the Golden Throne at his request. Here he would remain for the next 10000 years.
The sect of the Silver Sentinels that had refused to follow the remnant returned at this point, crushing a significant portion of the Chaos fleet by surprise, and carrying with them a massive number of fanatically devoted Emperor worshippers, who were understandably outraged at the betrayal and wounding of their deity by his own son. Many of these Emperor worshippers would form the backbone of the Ecclesiarchy that arose shortly after the Emperor was interred on the Throne, and, despite being able to speak to his followers with a text-to-speech device that he'd installed shortly before the Webway Gate was destroyed by a possessed Eldar turning into a Daemongate thanks to some cooperation between Tzeentch and Slaanesh, he allowed this, almost entirely to have an alternative religion to counter the one devoted to Chaos, since pure logic and reason had failed. The extra power it provided him was also of benefit, as it would help stave off his soul being ground away by the strain of holding the Astronomicon alight and keeping Terra from being dragged into the Warp by the broken gate.
The Adeptus Sororitoras was founded shortly afterward, mostly composed of members of the same group of fanatics, and created from a core of Silver Sentinel Legion serfs who, while not Space Marines, had completed punishing physical training regimens necessary to qualify for the treatment, and trained entire regiments of fellows fanatics to a similar standard. This was meant to give the particularly fanatical devotees an organized outlet, though it was unofficially the Ecclesiarchy's militant arm within 2 decades, and officially so within a century. The Sororitoras went on many of the same missions as the green Space Marines, fielding much of the same equipment, for both the Inquisition, and the general extermination missions regularly carried out against, for instance, the Orks. For all the Emperor's protestations of divine status, faith had power, and immensely powerful belief strengthened many of the Sororitoras beyond their already inhuman training. The shards of his soul empowering them merely boosted this effect.
The Silver Sentinels, much like the Ultramarines, believed that their Primarch would one day return, even if they didn't have a physical body to cling to. Of course, the nature of shadows and remnants is such that the original is reconstituted in the same place where they were split, meaning the depths of the Warp in this case. Most who know this suspect that Serras was torn apart by the Warp's whims, her reconstruction impossible in such a chaotic dimension, or fell into the hands of one of the four 'normal' Chaos gods, too tainted by Darkness to be corrupted, but too potentially useful to discard or destroy. Most of the latter suspect Tzeentch to be holding her.
Sanguinis acts as Warmaster, as Horus had once advised, before the family of demigods was shattered, safeguarding the Imperium against the myriad threats that arose and quelling any rebellions, though the Silver Sentinels kept the latter from getting as out of hand as they might have during the Heresy itself, the Legions were not allowed to recruit back to their old numbers, to make a second Heresy more difficult, leaving them spread thin.
Roboute's Codex barely managed to win the majority of his brothers' approval, or at least apathy in Dorn's case. The Silver Sentinels mostly took the Codex as a strong suggestion, but acknowledged that the smaller groups of Astartes would cover more 'ground' and raised little fuss in most cases. They were always an individualistic Legion, so while they and the Ultramarines were both generalists, many chapters of Silver Sentinels would vary incredibly widely, especially the eldest.
Overall, while things were certainly not excellent, they could have been much worse. The forces of the Imperium remained in more or less a deadlock with those seeking to destroy it, neither gaining nor losing ground.
AN: Yeah, I was running off... no sleep when I wrote this, but I think it's not terrible. Magnus stayed loyal, but they grabbed Lion instead. Only half his Legion though. Sanguinis survived, Emps was just distracted by the whole 'Malice is trying to kill the sun!' thing and preventing him from doing that took enough attention that Horus could sucker punch him. Ollanius and co. bought him enough time to fend off Malice and the moment he could actually focus on him, he ended Horus.
