Founded by the Explorator Arks sent out by the ancient Mechanicum priesthood of the Forge World of Phaeton during the Great Crusade in the late 30th Millennium, Atar-Median was quickly transformed into a fully functioning Forge World.
At the time of the Horus Heresy, Atar-Median was only one amongst several lesser Forge Worlds that formed the mighty "Belt of Iron" which spanned the border regions of both the Segmentum Tempestus and the Segmentum Pacificus.
Atar-Median is the homeworld of the Titan Legion called the Legio Atarus, also known by their cognomen of "the Firebrands," which includes many ancient god-engines that once defended Phaeton during the Age of Strife.
The seeds of Atar-Median's ill-repute during the Great Crusade can be traced to the highly unusual circumstances of their inception. In the early years of the Great Crusade, one of the first tasks of Imperial expansion from Terra was to link up with a number of key domains within the Segmentum Solar identified as vital to the Great Crusade's success.
In many cases, these inclusions went without difficulty, such as with Voss, whose masters had bent the knee to Mars without qualm. But in other cases, such as that of Phaeton, inclusion was a matter of some negotiation and difficulty, and the final acquiescence bought with considerably more autonomy than the fabricator-general of Mars would perhaps have preferred from a vassal-domain.
What began as a coldly cordial relationship quickly grew into one of veiled and open rivalry, with Phaeton's position being further strengthened by it quickly becoming perhaps the second most productive Forge World of the Great Crusade until the late rediscovery of mighty Anvilus.
Although never faltering in its commitment to the Great Crusade, machinations by the authorities of Mars brought matters close to outright conflict over doctrine and subtle accusations that Phaeton was building its military assets to threatening levels, and this began to have a corrosive effect on the Forge World's reputation -- to the point where the storm clouds of hostile censure and perhaps even armed retribution gathered.
At last, the ruling revered-comptroller of Phaeton Prime, realising that his domain could not stand before such power, brokered a compromise, offering publicly to give up a full third of his world's Titans, armed forces and chattels in the interests of political equanimity with Mars.
This was quickly agreed, but as the fabricator-general, no doubt with some lust for the task, despatched an assayance task force to Phaeton, he discovered that he had been outmanoeuvred.
Rather than give up its lore and dominion to Mars, Phaeton had created a fleet of vast Explorator Arks, and into these were divided an equal third part of its priesthood, chattels and machinery, and even as the Martian emissaries looked on, despatched them into the void.
Conflict was averted, Phaeton diminished and its subservience was assured, but it and its allies gained a long-burning antipathy toward Mars and its masters. After a long and perilous journey through the Warp, the Phaetonite Covenant alighted in the vicinity of the white super-giant star Atarath, located in the Segmentum Pacificus, on what was then the edge of known Imperial space.
Here they founded the fledgling Forge World of Atar-Median in its shadow, on one of the many rogue planets caught in the colossal star's gravitational pull.
With frightening swiftness the transplanted magi rose up their domain of steel and fire, and using as its core ancient god-engines taken from the forces that had once defended Phaeton in the anarchy of Old Night, it founded the Legio Atarus as its shield and sword.
The Dispute of Iron
Located across the border sectors of the Segmentum Tempestus and Segmentum Pacificus were numerous lesser Forge Worlds, which included Atar-Median, known collectively as the "Belt of Iron."
For Atar-Median, the Horus Heresy started on 566.006.M31 when the War Maniple Red Naga of the Legio Atarus was destroyed on the black sand of Istvaan V during the Drop Site Massacre. Whilst it is unknown if the magi of Atar-Median had been approached by Traitor-aligned Dark Mechanicum envoys as is known to have happened in the Segmentum Obscurus, this sole act of aggression would suffice to propel Atar-Median into the Loyalist camp.
Given the tension that existed previously between Phaeton and Mars, it could well be that the Traitor Kelbor-Hal, the fabricator-general secretly in league with Horus Lupercal, had deemed Atar-Median incorruptible and more efficient to subdue other, more-pliable Forge Worlds as not to waste resources on Atar-Median.
Blockading or cutting off domains they could not readily subdue was, after all, almost as efficient as destroying the enemy itself. By 010.M31, Atar-Median was fully at war, the famed Belt of Iron having succumbed to infighting in the Mechanicum as had much of the Imperium of Mankind.
Allied to the Loyalist Forge Worlds of Graia and Arl'yeth, Atar-Median stood against those worlds of the Mechanicum that had thrown their lot in with the Traitors: Incunabula, Urdesh, Valia-Maximal and Kalibrax.
The Belt of Iron's two remaining Forge Worlds, Arachnis and Jerulas Station, had both fallen into anarchy and full-fledged civil war and only marginally interfered in the internecine Mechanicum conflict in the region which would later be named "The Dispute of Iron."
In the years that followed, these Forge Worlds' Taghmata's oathbound Knight Houses and Titan Legions would devastate scores of human-inhabited worlds in the region that became the battlegrounds upon which the Dispute of Iron was fought.
Atar-Median has also been called Atar-Meridian, most notably in The Horus Heresy - Book Six: Retribution, page 274.
It is unclear whether the name of the world has actually been changed, or whether this is just an alternate form of the name for Atar-Median.
- The Horus Heresy - Book Two: Massacre (Forge World Series) by Alan Bligh, pp. 156-157
- The Horus Heresy - Book Six: Retribution (Forge World Series) by Alan Bligh, pg. 31