Cthonia

Cthonia was the barren and impoverished Imperial 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 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 Chaotic 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 Great 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 Dark 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.

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 gangs inhabited the lawless depths of Cthonia, enjoying freedom from the rigours of Imperial citizenship while at the time of the First Founding providing an easy source of human specimens for experimentation in the creation of the first Astartes that nobody would miss. One report talks of so-called Imperial "recruitment squads" rounding up thousands of Cthonian gangers and shipping them away, chained together in the holds of prison-shuttles, to the geno-laboratories of Luna. Here they were modified with the gene-seed derived from the genome of the Primarch Horus and became the XVI Space Marine Legion, later named the Luna Wolves when Horus took command. 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.

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, which he renamed the Luna Wolves - 10,000 Space Marines who were the true Sons of Horus. With these warriors to lead, Horus accompanied the Emperor for the first thirty years of the Great Crusade, and together they forged the initial expansion of the young Imperium of Man.

The Horus Heresy
Eventually, a great schism known as the Horus Heresy occurred. This was the first and most devastating civil war in Imperial history. Occurring in M.31 and lasting less than a 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.

Cthonia's Fate
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.

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 Imperial Navy ships that rebelled during the Horus Heresy now seem to be under Abaddon's command, along with newer vessels he has ordered constructed.