The galaxy is a vast spiral, ninety thousand light years across and fifteen thousand light ears thick, containing four hundred billion stars. Only a fraction of the stars have habitable planetary systems, and only a tiny fraction of these have been investigated by humanity or any other spacefaring race.
The initial human colonisation of the galaxy lies in the distant past, separated from the present by twenty thousand years of regression and rebuilding. Human worlds are' scattered throughout the galaxy but their distribution is not even. The greatest density of human worlds is in the galactic west, close to Earth. In the galactic east, in the area known as the Eastern Fringe, human worlds are few and often far apart.
Many human worlds benefit from mutual contact and a comparable level of technology. Others have become primitive and barbarous, often as a result of periods of isolation. New human worlds are being discovered all the time, and there remains an unknown number which have been isolated and forgotten for hundreds, if not thousands, of years
One of the millions of planets that are a part of the galaxy spanning Imperium is Necromunda. It was settled 10 000 years ago before the Horus Heresy and the ascension of the Emperor to the Golden Throne. It was a planet rich in resources that have been mined, harvested and stripped from the planet since. Its metals and minerals are now exhausted. Its flora and fauna have had their genomes mapped and filed and have since died in the toxic waste left by the encroachment of humanity upon the planet. Millenia of industry has reduced the mountains to rubble and the seas to sludge. Huamnity has had to retreat from the pollution it has created behind huge sealed Hives. These are cities that have built up over this period, upon the ruins of the previous century.
Necromunda is a dying world. It orbits an old moribund star, a Red giant, on the outer edge of the galactic north. The temperature outside the sealed Hives is below zero, always freezing or frozen. Necromunda is too far away from its sun to receive any real warmth, and despite the clogging clouds above, the temperature is never high enough to allow life to live in comfort - if it could live at all. The chemical rivers and sludge sometimes freeze over into huge, icy mirrors which the Tech-Nomads may sometimes safely cross. The planets rocky crust is scarred by the shifting corrosive ash that is the ground down waste of eons. Water is found only in deep wells, geothermal steam or the bodies of humanity. The atmosphere is poisonous and irradiated by 10 000 years of toxic gas and waste from the Hives and the constant deadly solar rays from the Red Giant. No life lives there without the aid of high-tech., life support or sealed vehicles.
Necromundas natural resources are exhausted. The Hives survive through constant re-cycling of wate materials. There is little left on Necromunda that is useful to the Imperium - except the slag factories that turn waste or recovered metals into ammunition and the roving gangs that make Necromunda's brutalised people born warriors for the Imperial armies.