Cthonia was the barren and impoverished Imperial Feral World and former Mining World on which the Primarch Horus was raised when his gestation pod crash-landed there after being stolen and transported through the Warp from the Emperor of Mankind's gene-laboratory deep beneath the Himalazian (Himalayan) Mountains on Terra by the Ruinous Powers.
It was also later the homeworld of the Luna Wolves Legion of Space Marines before the Horus Heresy, which became the Traitor Legion of Chaos Space Marines known as the Sons of Horus and later, the Black Legion.
Cthonia was destroyed some time after the end of the Horus Heresy, either by the forces of the Imperium who sought to destroy any trace of Chaos taint in the Emperor's shattered realm or because the world collapsed as a result of the geophysical instability caused by millennia of overmining within the planet's crust and mantle.
Birthplace of the Arch-Betrayer
The world of Cthonia existed in one of Terra's closest neighbouring star systems. Being within reach of even non-Warp-capable spacecraft, Cthonia had been colonised, built upon, tunnelled and mined probably since the dawn of human interstellar space travel before even the Age of Technology had begun. As such, all of the world's natural resources had been stripped away and used millennia before, and the planet's ancient mining technology had long since been rediscovered and removed by the Tech-priests of Mars.
The planet that remained was largely redundant and abandoned, completely riddled with catacombs, crumbling industrial plants and exhausted mine-workings. None now know who were the masters of this hell world before it was rediscovered by the Imperial forces during the Great Crusade. Some Imperial scholars speculate that it was the Priesthood of Mars, ever greedy for raw materials to feed their forge cities.
Other sources indicate that it was a star kingdom that dwindled to nothing long before Unity was even a dream on Terra. No matter who had once controlled the world, they ate the heart of Cthonia until it was a dead husk. Afterwards, before even the coming of the Age of Strife, perhaps, Cthonia had become an orphan world, abandoned to entropy and violence, and even before the great collapse, true darkness had descended there.
Whereas the early history of many Primarchs is extensively if unevenly documented, the same cannot be said of Horus. Contradiction and omission tarnishes all accounts of Horus' formative years. It is clear that the Emperor did find Horus and also that he took command of the XVI Legion early in the Great Crusade. Beyond these manifest facts, agreement between sources is decidedly lacking, some even placing Horus on Cthonia as a foundling.
Like many of his superhuman brethren, these sources say that the young Primarch thrived in Cthonia's harsh environment, learning his first lessons in war and killing from Cthonia's tech-barbarian kill-gangs. Another source claims that Horus returned to Terra itself.
It is said that Horus grew at the Emperor's side, learning from his father even as they took back the Sol System and forged the alliances between techno-barbarian nations and with the Mechanicum of Mars that created the Imperium of Man. Other highly creditable claims state that the Emperor found Horus, the first of His lost sons, but neither source specifies where, or the location of this finding. Surrounded in millennia of myths and allegory, the truth of Horus' origins will more than likely never be known.
Law of the Blade
Faced with complete and total economic and societal collapse, the people of Cthonia had either left if they possessed wealth enough to return to Terra or had sunk ever deeper into a terrible poverty defined by both its savagery and a desperate struggle for survival. Fierce, lawless gangs inhabited the depths of Cthonia, enjoying freedom from the rigours of Imperial citizenship.
There was no law but the blade, no desire save that to survive. Some gangs were territorial, their leaders possessing all the pretensions of barbarian kings. With armies of men and women bonded to their service, they would seize access to tunnels, demand tribute from other factions and create enclaves in the lightless heart of abandoned tunnel networks. To other gangs blood and power was a crop to be harvested by violence and violence alone, and the dead meat enough to live on.
Holding no territory and living from plunder, these gangs raided, murdered and burned. Where they did not need food, ammunition or supplies they would raid simply to enhance the fear they spread, or winnow out the weak and the undeserving from their own ranks. While these reaving gangs left blood and ruin at their passing, others moved like spectres on the edge of sight, killing silently and for ends that few could understand.
Between these factions a fluid web of respect, tribute and rivalry existed. Factions would form, evolve and dissolve in a few solar months. Of those that endured longer, only one thing was certain: their time too would pass. And so it was for the long years of the Age of Strife: the strong killed the weak only to be killed themselves as others rose up again and again and again, and somehow this murderous strain of Cthonian humanity not only survived, but thrived by murder and prospered by plunder, and so Cthonia endured for long years.
A Human Harvest
By the time the Great Crusade began in ca 800.M30, Cthonia's mines were long spent, but it had a resource that the new Imperium needed more than metals and jewels: hardened fighters and born survivors in their millions; a lean and hungry race of killers with no illusions about the horrors of the universe. Cthonia, relatively close to Terra in the void, and with whom some minor intermittent contact had been maintained even through the Age of Strife, had its murderous and strife-torn population marked by one of the first Expeditionary Fleets to leave the Sol System.
To fuel the growth of the early Space Marine Legions, the Imperium took full advantage of the bounty that Cthonia provided. At the time of the First Founding, Cthonia helped provide the necessary flesh for the gene-wrights of Luna to fuel the growth of the Legiones Astartes. One report talks of so-called Imperial "recruitment squads" harvesting tens of thousands of Cthonian gangers and shipping them away, chained together in the holds of prison-shuttles, to the geno-laboratories of Luna.
The majority were impressed troopers for the Crusade's Imperial Army regiments, but the finest specimens were taken for induction into the XVI Legion, later named the Luna Wolves after the first campaign beyond Terra in 798.M30. On Luna these chosen sons of Cthonia were transformed and reborn as the transhuman Astartes of the XVI Legion.
It was more common for the first Space Marine Aspirants to be volunteers from Terra, and later, after the rediscovery of the Primarchs, from Feral or Feudal Worlds. However, after the usual hypno-psychological indoctrination process, the Luna Wolves created from the men of Cthonia emerged as excellent and ferociously powerful Space Marines. Within the Imperial Army, the Cthonians developed a very similar reputation which led to the formation of several elite Solar Auxilia Cohorts, collectively known as "Cthonian Head-hunters", which would all side with the Traitor forces of the Warmaster during the Horus Heresy.
The Emperor Arrives
Information about the early days of Horus himself is even harder to uncover in Imperial records. It is thought that he was the first of the Primarchs to be recovered by the Emperor, having been cast much closer to Terra than the others, and was found at a much younger age. As a result, Horus was for many years the Emperor's only son, and there was a great affinity between them.
The Emperor spent much time with His protege, teaching and encouraging him to reach his full potential and a love sprang up between them that the Emperor believed nothing could ever break. Horus was soon placed in command of the XVI Legion that had been created from his DNA and in many cases from his own fellow Cthonians, who had already earned the moniker of the Luna Wolves during the Pacification of Luna in 798.M30-- 10,000 Space Marines who were the true Sons of Horus and would later bear that name as a Legion. With these warriors to lead, Horus accompanied the Emperor for the first thirty standard years of the Great Crusade, and together they forged the initial expansion of the young Imperium of Man.
Eventually, a great schism known as the Horus Heresy occurred when Horus betrayed his father and entered the service of the Ruinous Powers of Chaos in an attempt to usurp the Emperor's place as the Master of Mankind. This was the first and most devastating civil war in Imperial history. Occurring in the early 31st Millennium and lasting less than a solar decade, it divided and nearly destroyed the fledgling Imperium. It marked the end of the Great Crusade and the death of Horus and the Ascension of the Emperor of Mankind into the Golden Throne.
The Sons of Horus Legion that has been the Arch-Betrayer's own fled under the command of its First Captain, Ezekyle Abaddon, into the Eye of Terror, alongside most of its fellow Traitor Legions. There, over the next 10,000 standard years, Abaddon twisted the XVI Legion into a dark, twisted version of its former self that he renamed the Black Legion, the most potent of the Traitor Legions which serve the Dark Gods of the Warp.
In the 41st Millennium, the Black Legion's homeworld of Cthonia no longer exists, having apparently lost geo-structural integrity and broken apart into asteroids and debris during the centuries following the Heresy. Certainly the once ore-rich planet was riddled with mine workings right through to its dead core (in fact the numerous gangers that formed the population may originally have been imported as work teams to maintain the crumbling tunnels), however there is much conjecture that Cthonia was destroyed deliberately by the Imperium after the Heresy during the Great Scouring to remove the tainted world from the demesne of the Emperor.
Since the destruction of their fortress in the Eye of Terror, the Black Legion is no longer based on any particular planet, instead stationed permanently on various spacecraft. They possess a single ancient Battle Barge from their original fleet, as well as other vessels commandeered or captured over the years. In particular, many former Imperial Army warships that rebelled during the Horus Heresy now seem to be under Abaddon's command, along with newer vessels he has ordered constructed within the Eye of Terror by the Hereteks of the Dark Mechanicus.
- Index Astartes IV, "Sons of Horus - The Black Legion Space Marine Legion"
- The Horus Heresy - Book One: Betrayal by Alan Bligh, pp. 66-68
- The Horus Heresy - Book Four: Conquest by Alan Bligh, pp. 81, 94-95
- Horus Rising (Novel) by Dan Abnett
- False Gods (Novel) by Graham McNeill
- Galaxy in Flames (Novel) by Ben Counter