Adeptus Mechanicus - Mars: The birthplace of the Adeptus Mechanicus was the ancient Forge World of Mars. Mars was colonised very early in human history, long before the start of even the Dark Age of Technology, and developed a society different from Terra both culturally and in terms of technological advancement. The arid, rusty surface of Mars was terraformed, and under a man-made oxygen-nitrogen atmosphere, the Martian colony flourished, though it remained politically independent from Terra. When the era of human interstellar colonisation began, both Terra and Mars served as the co-equal mother worlds of countless new human colonies across the galaxy.During the Dark Age of Technology, the two empires of Terra and Mars co-existed under the aegis of the decentralised human interstellar government of that era, to the mutual benefit of both. At the height of its splendour during that era, and even later in the anarchic Age of Strife, Mars despatched hundreds of colony fleets into the void. Many perished in the terrible Warp Storms that engulfed the galaxy at that time before the birth of the Chaos God Slaanesh, but others survived or made their way forward at sublight speeds in realspace. Those who did founded new Forge Worlds in the name of the Machine God and built on them a likeness of the great manufactorums and temples of their distant homeworld. These Forge Worlds also forged new empires for the Mechanicus among the feudal Knight Worlds and added their unique combat walkers, the Knights, to the Mechanicus' own powerful arsenal.The onset of the Age of Strife brought an end to the glory and peace of the ancient interstellar human domain. Across the galaxy, Mankind suddenly turned upon itself as a new breed of Warp-attuned psykers emerged and Warp Storms engulfed the galaxy, rendering interstellar travel and communication dangerous to the point of impossibility. Civil war engulfed thousands of human worlds, even the twin human homeworlds of Terra and Mars.Many warring factions vied for power on Mars and waged a brutal civil conflict using arcane and unimaginably destructive weaponry, including psychic abilities. The carefully constructed atmosphere of Mars was burned away, and once more the rusty surface of the planet was exposed to the deadly radiations of the Sun. The terraformed Martian environment, which had teemed with Terran life for centuries, was rendered a barren crimson desert again. The Martian combatants now fought in the same blood-hued wastelands that the first human colonists from Terra had found millennia before. Much of the Martian population retreated underground, as any who were not equipped with a radiation and life support suit could not long survive on the Red Planet's harsh surface.In time, one faction emerged dominant over all the others that had contended for dominion over Mars -- the Cult of the Machine God, the Mechanicum. Believing deeply in a strange theology which held that the expression of the universe's divinity, the Omnissiah, was embodied in advanced technology and machines, the Cult Mechanicum established its rule over the Red Planet and erected massive new manufactorums and hive cities across the world, divided into different city-states known as forges or forge cities. Each forge was ruled by a high-ranking Tech-priest, and the entirety of the Mechanicum's hierarchy bowed to the high priest known as the Fabricator-General.For millennia the Mechanicum ruled over Mars, sending out colony expeditions to spread its faith and its way of life across the galaxy, which led to the founding of the first Forge Worlds. Mars also made war upon the divided techno-barbarian states that ruled Terra at this time, as many of these ignorant savages had access to advanced technologies left over from the Age of Technology that they did not understand or know how to venerate properly. This innate hostility between Mars and Terra lasted for centuries until in the late 30th Millennium, following his victory in the Unification Wars on Terra, the Emperor of Mankind came to Mars and landed atop the great volcano of Olympus Mons.At their first sight of the Master of Mankind many Tech-priests were overcome with the feeling that they had met the living embodiment of the Omnissiah, the Machine God clad in human form. In return for peace between Terra and Mars, the recognition of the Tech-priests' right to practice their faith and their right to maintain control over both Mars and the other Forge Worlds they had settled, the Mechanicum agreed to build Imperial starships at Mars' Ring of Iron orbital fleet yards and provide all of the war materiel necessary for the Emperor's Great Crusade.In addition, the Mechanicum agreed to be bound by the Emperor's commandment to never develop certain forbidden technologies such as artificial intelligence. Though the Mechanicum had no knowledge of it, it was artificial intelligence that had nearly destroyed humanity once before during the rebellion of the Men of Iron in the Dark Age of Technology and the Emperor was determined not to repeat history. With the signing of this agreement, known as the Treaty of Mars or the Treaty of Olympus Mons within the Mechanicum, the Cult Mechanicum became the Adeptus Mechanicus of the new-forged Imperium of Man, and the Imperial Aquila was refashioned to bear two heads, one for Terra and one for Mars.Yet the terms of this agreement did not sit well with some within the hierarchy of the Mechanicum. A minority of Tech-adepts believed that the Emperor was not the Omnissiah, for the Machine God that the Mechanicus had always worshipped actually lay sleeping deep beneath the crust of Mars in the Noctis Labyrinthus. These Tech-priests particularly chafed at the end of Mars' political independence and the imposition of the Emperor's restrictions upon what technologies the Tech-priests of the Mechanicum could research and develop. As the Great Crusade progressed into the early 31st Millennium, these simmering discontents would ultimately cleave the Mechanicum just as it would the wider Imperium.During the Horus Heresy many Mechanicum units declared for the Warmaster Horus and fought against those of their faith who remained loyal to the Emperor, becoming known in later times as the Dark Mechanicus. These Traitor Mechanicum units initiated a civil war on the Red Planet known as the Schism of Mars that mirrored the larger conflict raging across the galaxy. The Fabricator-General himself sided with the Warmaster, but his deputy, the Fabricator Locum Kane, remained loyal to the Emperor.Horus swayed the Fabricator-General of the time, Kelbor-Hal, to the side of Chaos by promising him a complete STC database from the planet of Drakonis-Three-Eleven and brought the senior Tech-adept in his 63rd Expeditionary Fleet named Regulus over to his side with other STCs recovered from the Imperial conquest of the Auretian Technocracy. Horus promised much else in terms of technological knowledge to the Mechanicum, including the right to develop technologies like artificial intelligence previously forbidden by the Emperor in the Treaty of Mars in return for their allegiance to his cause.Many Master Adepts like Kelbor-Hal longed to pursue these restricted lines of research and had long believed that the Emperor was not the Omnissiah of the Machine God, for the true Machine God dwelled in the Noctis Labyrinthus deep below the crust of Mars. That this legendary creature may actually have been the C'tan shard known as the Void Dragon is a disturbing possibility.