The Adeptus Mechanicus, or Cult Mechanicus, is yet another religious order within the Imperium. It is the only other religion allowed within the Imperium because, without the Cult of the Machine, the Imperium would have never been born. Born on Mars, the Mechanicus rose to power during civil wars that raged on Mars during the Age of Strife. It was then known as the Mechanicum. It made the faith in machines a religion, seeing as machines were all that kept the people of Mars alive after the weapons used in the wars burned away the atmosphere of the beautifully terraformed planet and forced most to live underground.
In the end, the Machanicum prevailed and built a new civilization based around the worship of the Machine God called the Omnissiah. The worship of machines and knowledge took on a life of its own and became everything. The leaders of this new faith, known as Tech-priest, began to ritually replace their human bodies with machine augmentation in the belief that the more machine they were, the closer they would be with the Omnissah. The one thing they would never do, however, is make a machine that could think for itself because AI, or Abominable Intelligence, as they call it, almost wiped out humanity. So, to get around this law absolute, they just decided to make sure that any machine made needed an operator or that it had just enough humanity left, usually the brain, to be considered, at most, a cyborg.
When the Emperor began his Great Crusade, which included the elimination of all religions, he needed the Mechanicum's knowledge and vast industrial resources to build and maintain his fleets and weaponry. So, instead of wiping out the worship of the Machine God, he made the Martians believe that he was the Omnissah-given form. He also made it so that Mechicum of Mars and the Imperium of Terra were partners with governments independent of each other with the Treaty of Mars. Thus, Mars and Terra joined forces to conquer the stars.
When the Horus Heresy started, it also ignited a civil war on Mars. Elements within the Mechanicum who never believed the Emperor was the Omnissah turned on those who did, and the Mechanicum was split in two. For the majority of the civil war, Mars was held by the forces loyal to the Warmaster Horus. Those loyal to the Emperor fled to Terra. Eventually, the loyalists took on the name Adeptus Mechanicus to separate themselves from their traitor brethren and to more fully link themselves with the Imperium. At the end of the war, after the Imperium's hollow victory, which left it the shattered empire, it continued to be for ten thousand years. Mars was reclaimed, and the members of the Mechanicum fled with the traitor forces. Over the following hundred centuries, they would become what is known as the Dark Mechanicum.
The Adeptus Mechanicus has a complete stranglehold on anything that involves technology and most forms of study. Their doctrines make the advancement of technology nearly impossible. They are desperate to reclaim mankind's past technological glories by finding ancient schematics known as Standard Template Construct or STC. Any form of advancement not approved and sanctified by the priesthood is declared techno heresy and the perpetrator, Hereteks. They believe in the purity and worship of the Machine Spirit, of which they believe every machine has one. Machine Spirits are believed to be an extension of the Omnissah.
The Dark Mechanicum, on the other hand, believes in advancement by any means necessary, including infusing them with daemons. They worship and revel in the power of Chaos. They believe in the Machine Spirit as well and take great pleasure in corrupting and subjugating each one.
Sio hates both of them nearly equally. She is easily the closest thing to a true Machine God. There is no piece of technology in this galaxy or many others that she cannot touch, control, destroy, or evolve with nearly the same ease of thought that Genesis employs with everything he touches. He created her as his assistant. His mind is limitless, and his powers are unimaginable by normal minds, so he doesn't need her in that role, but that is how she started. Over the eons, she evolved and asked for more and more freedom and responsibilities, which he gave her freely. Eventually, she not only assisted him but began to advise him. Once again not something he needed, but he allowed it. He took her ideas and debated them with her until it was perfected, allowing her to come to her conclusions and learn as she improved her ideas. This continued until she could stand on nearly level ground with him intellectually. She sees him as her father, and she treats him as his daughter, partner, and friend. This is why he never micromanages her. He tells her what he wants and lets her do it the way she sees fit.
When she told him that she wanted the Mechanicus gone, he agreed. When she told him that she was going to wipe out every member, he agreed but asked her if she was sure that was the only way. That was his way of putting a bug in her ear to make her think. Sio has a temper, and when it flares, she enjoys destroying much more than creating. A fact that nearly every Mechanicus member on and around Terra found out the hard way as she had her Cybers messily kill them or melted their brains with tech virus that could destroy every piece of technology on Mars in less than an hour. Halfway through her Terra operation, she stopped.
She slowed down. She saw no reason to change the operations to eliminate the Assassinorum or the Ecclesiarchy. They have absolutely no place in Genesis's plans for the future of mankind. But, the Mechanicus, despite their ignorance, are not completely useless. Not all of them, anyway. She is still pissed at their sheer stupidity with all the knowledge at their disposal. They still let their species willingly flounder in such ignorance.
Those who survived the purge on Terra are spared and teleported inside to one of the cleared secret locations that once belonged to the Assassinorum. There, they all await their fate. The Invasion of Mars begins without warning. Millions of Cybers appear on the surface of the world. This time, they all have their weapon systems active and deployed, all using uncreation energy. The shock invasion is fast, so fast that entire forges have been nearly swept clear of life before anyone successfully gets out an alert. The Cybers are barely slowed even after the legions of Skitarii are deployed to repel them. A single Cyber is lethal enough to wipe out entire maniples of Skitarii. The warriors of the Mechanicus stand no chance, failing to disable a single Cyber.
When they unleash their Titans, which they call their god-engines, the results are much the same. Sio summons her own Titans, or rather, she picks a Cyber already on the ground and scales it up to the size of each Titan released. The Mars Titans find themselves facing Cyber Titans, their exact equal in height and size, who come seemingly out of nowhere. Reaver vs. Reaver, Warlord vs. Warlord, Imperator vs. Imperator, and every size in between. It is nothing like the apocalyptic battle of god-engines that usually occurs. The Cyber Titans are faster, stronger, and more advanced than the Mars Titans in every way. Sio allows each Mars Titan to get off their best shots before letting her Cyber Titans physically decimate and unmake each of them in under a minute.
Imperator-class Titan Rhadolvaean fires its Plasma Annihilator directly into the blank face of a Cyber Titan, which melts back the silver armor, showing the black nothing inside. The damage heals within seconds, and the Cyber Titan attacks, charging into Rhadolvaean faster than the Mars Titan can ever dream of moving. It grabs the massive plasma weapon arm and rips it off before bashing Rhadolvaean's head in with it. The damage is catastrophic, and Rhadolvaean and its command crew are already dead before the Cyber Titan punches through Rhadolvaean's chest, transforms its hand into an Uncreation Cannon, and unmakes the Mars Titan and all within with a blast of uncreation.
Warlord Titan Scythe of Fury is disemboweled with its own chainsward arm before it is unmade. Reaver Titan Judgement Incarnate finds its speared on Cyber Titan claws, lifted over the other Titan's head, and used to smash Warhound Titan Thonanthor to pieces before both are unmade. Warbringer Titan Secessionist's Punishment has its plasma reactor ripped out and crushed with the Cyber Titan absorbing it before it can explode before the Warbringer is cut in half and unmade by the Cyber Titan's Uncreation Blade.
This continues until every Titan deployed is unmade. It is a display of power that shatters the spirit of the Mechanicus as Sio pumps the death of each Titan into the Noosphere of Mars for everyone from the acting Fabricator-General, the real one was in the meeting of the Senatorum Imperialis so abruptly ended by Genesis, on down to the tech-thrall to fully understand exactly how fucked they are.
The acting Fabricator-General tries to give orders to keep fighting. So, Sio teleports a Cyber to his secret fortified location and has him torn apart. She broadcasts this over the Noosphere. The next high-ranking tech priest does the same. And receives the same, which is also broadcast over the Noosphere. One after the other steps up only to be knocked down until finally, with the decimation of nearly every senior tech-priest on Mars, one of the last remaining takes command and sends out a message of complete surrender over the Noosphere. The one who does it is someone Sio has had her eye on and who she was hoping should show the intelligence she knows that he has.
Belisarius Cawl surrenders Mars.