Chemos

No. It is not a shorthand for chemotherapy, although given the Primarch it gave birth to...

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?

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.

Eventual Fate

During 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, it was still being used as a supply point for the traitorous legion.

Sanguinius, Roboute Guilliman and Lion El'Jonson all decided that in order for one of their Legions to get back to Terra in time for the Siege, the remaining two Legions would have to strike at the Traitor's rear to create an opening. Thusly, the Dark Angels set themselves the task of burning the Traitor homeworlds and cutting off their supply lines. Chemos was bombed to dust before the end of the Horus Heresy even happened. However, it did not have the desired effect and the traitors only continued to press towards Terra in an all-or-bust strategy, forcing many in the First Legion to lose heart and causing the Lion himself to wonder if the war had already been lost.

In the aftermath of the Horus Heresy, Chemos's location and history have been hidden by the Inquisition and the world placed under quarantine. Its current status -if any- is unknown. (won't it be the greatest irony if it was now a Sisters of Battle retirement world? Though unlikely considering the high chance it was turned into a irradiated husk by the Dreadwing.)