Fabius ignored the urge to retch that was fighting against his Astartes nature. The warp null tore at his soul, causing his whole being to respond in hate to the individual he was performing the surgery on. The neophyte had been a casualty in his training, a man who had taken a shot to the temple. Most of his brain had been scrambled. A good apothecary would have taken the geneseed and left the man to the training grounds of Terra. However, he was not strictly dead and Fabius needed subjects, so like many before him, the fate of Subject-178 was set the moment Fabius came across the half-dead man.
He would not live as an Astartes; his brain was far too damaged, but he was proof of Fabius' ideas. A Pariah artificially induced, one made at great cost, and the chances of survival would be even lower for men who were not brain dead. However, it was evidence that Fabius had access to the knowledge and skills required to create a null. A marine's strength and power combined with an oppressive aura, a force to weaken mortals and to slaughter psykers.
His failures in that field had caused him great annoyance. While he had found the secrets behind the creation of a null, a far rarer creature than a psyker, the induction of psychic potential into a body had never succeeded. His current consideration was that it was because of the brain death of the subjects he had access to. The Selinar woman was back on Luna running tests with live mortals for comparison. The results so far were dim; if extrapolated to Astartes, only one in a thousand were even viable for the process and only a thousandth again could survive all steps. It was a high price, one he doubted his own legion would be willing to pay.
So Fabius reached out for aid through hidden channels to make his project a reality.
[][Fabius] The Selinar
While he would not have access to the power of Astartes, making humans into low-ranked nulls will allow the creation of the Null Auxilia, a weaker force than the Sisters of Silence but one that is far more numerous.
[][Fabius] The Emperor
It is a long process to have an audience with the Emperor himself, but Fabius is able to gain that honor. Guilliman is unwilling to sacrifice the men needed to make a force of Blank Astartes but offers a path. Fabius helps create a process for a new dreadnought that uses an Astartes Blank of his creation to become heavy forces against psyker positions.
[][Fabius] The Legion Master
In the end, the best one to go to would be his own leader. So he approached the legion High Command and spoke to him of his discoveries, his processes, and what they could offer. As he predicted, they would not agree to the use of this technology, finding it too grim and ineffective for its cost. However, with his research known, he gets more support for his projects with the psyker.
[][Fabius] A Sigillite
An agent, an unknown serf, had seen Fabius and watched him. He reported back, and an entire network slowly relayed the information to the spider at the center of the web. A man who is very interested in this project. The blind guide of the Emperor, the Sigillite himself, approaches Fabius.
The Legion did not have time for games; it did not have time for those who would spit on the gifts of the Empire. They had to be shown the ways that the xenos auxiliaries could be used. Commands were made, and the Legion began to limit their deployment of the Solar Auxilia elements with them and the other human elements. The captains and other legionaries are made to work with abhuman and xenos elements.
However, what quickly becomes apparent is the xenos do have a distinct difference in battle compared to humans. Where humans were largely the same and, outside of the largely standardized armor and weapons, did not vary much in abilities beyond simple skills, abhumans were specialized, largely having unique traits making them better in some roles than others. Xenos more so. While some were largely equivalent to humans or an abhuman in ability, the Xeno Auxiliaries could vary wildly in capabilities. Small crustaceans that piloted small swarms of automata. Lumbering shelled brutes the size of tanks with a sonic attack that paralyzed the enemy. Each Xeno could take a while to integrate into a larger command structure.
Ironically enough, the xenos auxiliaries have been performing worse than the human ones, a state of affairs caused by the lack of skills in the command of such a diverse force. The legion tackles this issue…
[][Children] Chapter Focus
The xenos will be categorized broadly into the roles that they can fit easily. Then each chapter of the Legion is tuned into commanding and directing these categories. The xenos auxiliary as they arrive will be sent to the appropriate chapter.
[][Children] Supreme Doctrine
This is a feat that would take a Primarch to truly reach—a unified codex of all possible ways to command mortal troops. While the Supreme Doctrine is not the absolute authority of war, the Legion hones their understanding of every possible xenos they could be in command of and how to use and integrate them into battle plans. Even if this consumes time that the legionaries would have used for other pursuits.
[][Children] Push Through
The legion continues to use the xenos as they can but does not put in extra effort into integrating them into battle plans beyond ones that can easily fit a quick idea of what they can do.
[][Children] Mortal Commands
The xenos auxiliaries are trained in their own styles of warfare, all the auxiliaries are. The legion will set up a looser command structure in the auxiliary cores, allowing them more ability to act in battle to their strengths.
Cassian Vaughn puts out the order; the legion responds, the ships leave, and a thousand men put shovels to the soil. Bunkers and trenches—it is not the traditional warfare of the XVIIIth, but the legion is able to adapt, to use their strength and knowledge, and in the time before the Eldar land on the world, a killing field has been laid down. The xenos warriors are a hard foe, their plans wily and their forces able to slip between the fire of a heavy bolter. Their elegant vehicles were able to dodge the cannon fire of the Legion's armor.
The trenches are a puzzle for their foes, though. Against a legion like the Iron Warriors or the VIIth, no Eldar would be able to slip around the lines. The XVIIIth were not a legion for siege warfare, so as the Eldar tested their positions and struck against the line, they slipped through, striking at the legion's back. Disrupting enough that despite the casualties, the Eldar advance is able to push against the Legion.
The legion's reinforcements have yet to arrive, but the Eldar are making pushes into the line. As each position falls to cunning and trickery as well as the nimble weapons of the enemies, the Legion had to change their tactics.
[][Flames] Pull Back
The current position is not tenable. Pull back to the more naturally defensible mountains. A dangerous gambit, but perhaps the only one with a chance.
[][Flames] A Prisoner's Use
Desperate times call for desperate measures. As the Eldar attack, the XVIIIth change their strategy, moving the captured civilians to the front of the battle line to hopefully blunt the attacks.
[][Flames] The Fire that Burns Twice as Bright
The line was held, but only just. Heroes of the legion were born, those that took on impossible odds, suicidal charges, and despite meeting their deaths, these torchbearers let the line be held.
[][Flames] Break The World
The Legion did not falter, the Legion did not break. The foes would be made to die; the line would be made to hold even if the lush world was turned to ashes.