Cthonia
Cthonia (meaning underworld or underground in Greek) was the homeworld of the Luna Wolves/Sons of Horus. Inhabitants of Cthonia had slightly freckled faces and pale, craggy features.
History
A mining world reachable from Terra at sub-light speeds (WHAT?[1]), it was mined out by the Age of Strife and stripped of the mining equipment by the Mechanicum of Mars. There was literally nothing left to do but organize into loose gangs and stab each other. Horus landed here, but didn't stay on the planet long enough to conquer or fix it. As a youth, he discovered what would be the Talon of Horus on the advice of an old crone, pretty much the only other interesting thing that happened on this world and thus naturally not explored in the fluff.
The first Imperial contact with Cthonia took place in the early Great Crusade, when a Star Hunters pioneer company under Captain Kornelius Dure discovered the world and declared it a "nest of serpents coiling in the dark that we would be better to destroy". Which was true, actually, but not for the reason he thought. However, the planet was revealed as the homeworld of Horus, one of the Emperor's lost Primarchs and the second to be rediscovered. Upon incorporation into the Imperium, the Space Marines of the Luna Wolves Legion were created using the human inhabitants of the violent gangs inhabiting the planet's Hives. The Emperor found him while looking for recruits of good gene-stock, and the planet was left in anarchy to help train future Legion aspirants.
During the Heresy, it was swiftly blockaded by the Imperial Fists, due to being a traitorous waspnest in the Loyalists backyard. The Sons of Horus still had a huge amount of materiel there, as well as a whole population of recruits and the facilities to turn them into Astartes; eventually the Imperial Fists began recruiting from Loyalist elements among the gangs, with the result of the Imperial Fists having more Cthonian natives in its ranks than the Sons of Horus relief forces sent to end the blockade. The Blockade of Cthonia ended near the end of the Heresy when advance elements of Horus's armada enroute to Terra tipped the balance in the traitors' favour.
Shortly after the Siege of Terra, it got Exterminatus-ed by the Dreadwing using a Vortex Bomb to split the planet in two, denying the Traitors their foothold in the Segmentum Solar.
- ↑ Well, Alpha Centauri is 4.36 lightyears from Earth, so assuming you can make roughly 80% of lightspeed, that's 5 and a half years one way trip, so it roughly checks out in 40k terms.