Chemos

Chemos was the homeworld of the Primarch Fulgrim of the Space Marine Legion of the Emperor's Children and by all accounts not somewhere you would want to live.

Chemos was an industrial world before the Age of Strife, and had traded its mineral resources for food from other worlds. However, when the Age of Strife began, Chemos got cut off from the rest of the galaxy at large and was forced to try to fend for itself. It was a losing battle, as they couldn't produce enough food resources, water and the like to survive and the population was in a slow decline to extinction. In desperation, all non-essential industries such as art, music and anything remotely creative were abandoned and everyone was focused on trying to get as much sustenance out of the barren planet as possible.

Grimdark enough for you then?

Seriously though, why didn't the Chemians think to construct bio-domes and the like on their planet to feed themselves while they had the chance during the golden years? You'd think common sense would tell you secure yourself first and then think of working on a wealthy trade system? They were trading for food well why not then for plants, stock animals and the like? But then that won't be grimdark enough so I should just stop thinking common sense. Anyway. It's not like there are entire countries in current era that can't produce enough food for themselves for at least decades (UK for example). Yeah, except those countries grew into that, Chemos was colonized back when humanity made the Forerunners look primitive. Besides that, there is a difference between needing the help of other countries/worlds and being entirely dependent on outsiders for your mere survival. Humans hate being reliant on others for survival. Countires that cannot feed their people in the lose of trade are capable of making do with reduced resources and likely mass starvation. Chemos had almost nothing to feed its people with without trade.

The decline all changed when Fulgrim speared down from the sky and smashed into the planet. Against the wishes of the ruling bodies of the planet the leader of the expedition to the crashed status pod decided to adopt the young Primarch as his own son and named him Fulgrim. Soon the Primarch grew at a great rate and did more than his share of work in the industry of the planet. He rose to become a leader of the industry and then one of the rulers on the planet. Deciding to fight back against the decay he saw he encouraged his fellows to brave reopening long lost mines and outposts to increase the productivity of the factories. Soon under Fulgrim's guidance, the planet was producing more resources than it needed and art and music began to flourish again as they were reintroduced to society. Chemos became a beacon of productivity and civilization again, and when the Emperor arrived, Fulgrim was all ready to join him on his merry crusade.

Not to put a damper on things or Fulgrim's accomplishments but it seems odd no one else on Chemos even though to carry out the idea to expand their industry. It's hard to believe that unless everyone on the planet was a few screws short of a lasgun. Maybe people were so busy working for food their minds have shut off to the possibility of increased productivity. Or that the labor requirements were so high for what they had, no one had time for new construction or R&D.

During the Great Crusade, there wasn't much word of what happened on Chemos besides regular influxes of recruits for the Emperor's Children came from the world. Considering it was a world of culture, music and relaxation now, it could be this had an effect on the recruits and left them more open to the corruption of Slaanesh, because at the time spiritual discipline wasn't something the Legions were tested for.

After the Horus Heresy, what with Fulgrim hooking up with Slaanesh and the Emperor's Children deciding to form their own ghastly version of a punk metal band, Chemos was bombed to dust by the Imperium for fear that the corruption of the Primarch had come from there, despite no evidence presented this was the case. As with the traitors' homeworlds in general, it was a case of "shoot first, ask questions never".

Since then, Chemos's location and history have been hidden by the Imperium and its current status if any is unknown (won't it be the greatest irony if it was now a Sisters of Battle retirement world? lol).