My imperial guard regiment is called the Vesuvian First Strikers.
Their planet Vesuv was a technologic powerhouse and very resourceful before the long night. When the contact to the imperium was lost the three continents established their own gouvernments and soon an all out war over the control of the planet broke loose. Every nation prooved to be equally strong so regardless of the horrible losses the population suffered it was pretty much a standoff.
That was until the lord gouverneur of continent secundus, Allan Zhimotheus Bixx gave the order to build a truly massive atomic bomb which was to wipe out continent primus completely. He anticipated continent tertius would give up out of sheer Terror and he'll end up being the governeur of at least two thirds of the planet.
The bomb was delivered via a carrier drone which flew a large bow into outer space to not be detected too early. Everything went according to Bixx' masterplan. Although when the bomb hit the soil of Vesuv, digging deep into the ground before exploding, things went south.
The bomb hit a weakspot in vesuvs crust and the following eruption blew up a vast amount of continent primus flinging trillions of tons of soil into the atmosphere and darkening the sky of Vesuv for decades with radioactive dust.
Only a few thousand citizens survived this incident hiding in the vast bunker complexes of the Vesuvian war. The planet turned from being a beautiful green and technological advanced planet into an icy, radioactive hell with no sunlight in just a few months. The long darkness became very true for Vesuv. The few survivors that were brave enough to climb to the surface years later found that the only place where it was possible to survive was near the gigantic lava lake which was formed by the eruption. It was the only source of heat on the planet at that moment. Also, the eruption destabilized the planet in itself. The tectonic activities went bonkers and new cracks which were up to a few miles deep opened and closed randomly on a daily Basis. The area around the red hot lake was ironically the most stable one on the whole planet.
Vesuvs only City was founded at that time when the survivors started settling around the lake. They built crude Habitats with airlocks out of random pieces that were scattered around the surface. Water reserves were a problem since most of the water was either vaporized or disappeared through the cracks of the crust of the planet. Explorator teams were out in the wastelands all the time to look for sources of water for the settlement. Hordes of Mutant raiders outnumbering the Humans by far were plagueing the whole wasteland. No one knew how they were able to survive at all but they were there. It was a constant fight for the survivors whose population didn't grow very quickly because of the constant birth of mutated or even dead offspring due to the radiation. The Vesuvians worked together as one. A strong bond between the People was formed that time.
The Sun eventually broke through the thick cover of dust after a few more decades. Uncovering a blinding white land of ashes. The population grew. The City grew. More and more lost technologies were ripped from the ash covered ruins of the past. Flying Caravans started forming to collect water from the poles where the ice crust was too thick to disappear into the ground. Explorator teams were on the everlasting mission to bring home new old stuff to build new stuff. The damned Lord Gouverneur Allan Zimotheus Bixx was never forgotten. In fact most Vesuvians used that Name on a daily Basis - as a curse. "Bixx!"
An animal which is only found on Vesuv contributed largely to the survival of the Human population. It is one of the very few species that survived. They were called "Rockeaters" because that was what they did. Large, friendly rock-eating Caterpillars, long as a schoolbus when adult. They were not that common before the Bixx-incident because of natural predators which ate their offspring so they actually benefitted from the Catastrophe. Rockreaters lay many eggs which are very nutricious and helped many People survive. In Addition they build their nests out of a secrete which the Vesuvians call "Rockeater Silk". This fabric has to be one of the strongest in the galaxy. Made of liquidized rock it can be harvested and woven into Cloth which feels like it was made out of steel wire. It can stop bullets and it isolates very well against radiation. To this day every Guardsman leaving Vesuv to fight for the Imperium is gifted a Beret made out of Rockeater Silk. It is lightweight but as good as a helmet. Any Vesuvian soldier would rather die than lose this piece of clothing.
As the Imperium arrived to reclaim the planet they found a flourishing and colourful City which looked like it was designed by a madman. The City (which was only ever referred to as - the City - because why would it need a Name if it's the only one on the planet) had a perimeter of roughly 30.000 miles with a depth of about 60 miles. Right around the still red hot glowing Lava lake which used to be the majority of continent Primus. The Vesuvians had established a Military gouvernment which was the only way to go because the Military was the only thing that kept everyone alive. A large majority of the Vesuvians was at arms. With just under a Billion Citizens Vesuv was not what you call a densely populated Planet and the Mutants which still populated the wasteland were an ever present danger to everyone. Nobody knew how the abhumans survived or recreated but apparently they did. They fought a constant war against the People of the City. Always looking for a gap in the high wall which enclosed it.
The Vesuvian Explorators recovered many of the old Techs. The Imperial Delegation was surprised to find that the Vesuvian soldiers used Weapons such as Lasguns of high Quality. Actually there wasn't much that they did not produce in the factories of the City. The Vesuvians were by no means poor and primitive at that time. The Laws of the Military Regime were strict and effective and just about everyone on the planet knew his place.
The leading officers of the Vesuvian Military knew there was no choice but to pledge to the Imperium. It was an unbloody annection and an imperial planetary gouverneur was issued to rule the planet in the Name of the Imperium, which not everyone liked because the word "Gouverneur" still had a bad taste for for everyone. The Imperial Gouverneur quickly learned about the potential of the planet. Vesuv still was loaded with resources. Heavy duty Equipment able to withstand the tides of the planet were issued and a few years later the planet was riddled with drop-in automated mining stations and basecamps full of soldiers to guard the stations and the transports. Under the Hand of the Imperium the Vesuvian economy grew vastly in a short amount of time.
At the beginning of the 41st millenium Vesuv had become a major arms and resource supplier. Since The City couldnt grow in width anymore it grew skywards. The City had become an absurdly large circle shaped hive city in which 99% of the human Population lived, worked and died. The Regiment of the Imperium was a hard one and there were many not satisfied with the lifes they were to live. Organized crime was a growing business. Many People decided to join raider gangs which lived a nomad life out on the rough surface. The Mutant Population was still present too, living in hidden spots in the cracks of the surface. It was quite impossible to rid the planet of them.
The Vesuvian Military as a result was always busy. Every human could join the Army on Vesuv. There were no Background checks. In the eyes of the Commanders joining the Army to fight and die for the glory of the Imperium was enough to rid everyone of their sins. Joining the Army basically meant you got a new life. Many Guardsmen from Vesuv are shady individuals. Probably coming from the Wasteland Raiders or one of the many Gangs of The City hoping for a better life in the Army. Training in the Vesuvian Army is hard. New Recruits were issued to guard Mining Rigs, Transports or to wipe out Mutant or raider nests in the Wasteland. No Guardsman leaves Vesuv without having prooved himself first. As a result Soldiers from Vesuv already are battle proven Veterans having earned their Rockeater Beret with sweat and blood.
... to be continued. Break is over

Sorry bout my "German English"