I've been an avid 40k fan since nearly the beginning. For a long time, I loved the game- strategizing, army-building, and rolling dice. Slowly, as I learned how to, I began to like painting more. Then I went to work for the Emperor himself and got into the aspect of teaching. But above it all, I have loved the fluff- the story, the background, the setting.
It's a dark universe. Warhammer 40,000 is gothic, teeming with danger, and science fiction taken as far as it can go. There's ALWAYS something big happening somewhere in the universe. All-in-all, there's no setting for any game system that is as in-depth or controversial (in that thought-provoking kind of way).
How does 40k do that? So few of the gamers have actually thought about the setting of 40k as a whole. Everyone knows the basics- in the grim darkness of the future there is only blah blah blah. Some, most of whom have played the Dark Heresy or Rogue Trader RPG, have thought about the setting in the more 'local' kind of way. But even then, everyone naturally thinks of the obvious- everyone worships the Emperor and life just sucks. But there's more, isn't there?
For this article, I'm gonna work up the scale of the Warhammer 40,000 universe. I want to show off just how detailed and rich the setting truly is, from bottom up. This won't be complete (20 years of monthly revisions and three RPGs have failed to do that so far), but it'll be a great attempt, and maybe even useful later. As always, let me know what y'all think!
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As a citizen in the Imperium of Mankind, there are some things that are canon. The planet you live on is part of a massive Empire that encompasses all the stars in the sky. There is a diety, named the Emperor, who lives on Terra and His vision has united ALL humans together for mutual survival. And your lot in life is one you cannot change. Fate and luck are the greatest tools in the Imperium.
This is the planet you aspire to. As a child you were serenated with stories of the glory of Terra. The blackened landscape, devoid of water or vegetation. The ENTIRE planet is covered in buildings, each built thousands of stories tall. Gantries, suspended miles in the air, allow millions to pass from one mighty building to another without ever descending floors. The sky is brown- long choked with noxious gases and barely breathable to un-enhanced humans. And religion is everywhere- this is the Throne-planet of the God-Emperor after all.You see, for ten millenia (and many before that), humans have been building and destroying society piece by piece. Terra may be completely preserved from any meaningful change for all history, but every other planet in the galaxy is oooooooold and has never remained the same. The planet you live on only has history stretching back three millenia, but it's been occupied by humans for four times that age. What happened only a thousand years ago is already legend and myth, corraborated only by archeological digs. Ancient buildings and long-forgotten technology are the only clues to things that happened before memory.
In all that time, mankind has been busy. They've built and rebuilt, demolished and restarted, and adapted to whatever the conditions heppened to be. In almost every case, each planet had a history of being verdant and beautiful. Thousands of years of development have destroyed that, replacing continental forests with massive arcologies. In those hundreds of centuries, humans have made the planets of the galaxy into something useful, utilitarian, and even spartan. And so have terms, such as 'verdant' and 'jewel', disappeared.
Because of this hostile galaxy, mankind has had to stretch beyond the mortal realm. You may never see a machine or medicine in your meagre lifetime but billions of humans unshackle their mortal bodies all across the Empire. On the most 'civilized' Capital Worlds, swathes of the population have enhanced their bodies with bionics. Many more have experienced the most base and advanced drugs mankind has ever developed, leading to inhuman dregs on one extreme or centuries-old Oligarchs barely recognized through all the mechanical parts.The mutating affects of poisonous atmospheres and lethal radiation from countless suns have affected mankind everywhere. Just as often as a human baby is born with all appendanges in the right place, so too is a human baby born with some mutation or degeneration. Most of these children are discarded immediately- some being lucky enough to be transformed into techno-religious artifacts. Worse, some mutations aren't visible. These humans turn out to be mankind's greatest threat and are yet undetectable except through horrific ritual and surgery. So do most 'Psykers' grow up to randomly manifest their powers- much to the danger of the local population!
Your planet, for the time being, is one of the most peaceful in the galaxy. You live a full life of five or six decades. Your towns house a few thousand people each, all trying to survive in a rustic wilderness of technology and colonialism. But this is almost unique. Where millions live in small communities, billions OF BILLIONS of souls live crammed into dirty, degraded, gothic cities. The scale of those cities is outside imagining. You may never see an Imperial Basilica, but most will never get to see the sky.
You'll never see the Emperor. But He's real. You'll never know why he's so important, but He is more important than anything else in the universe. It is this very faith that connects you to the stars. As a citizen, you are not taught philosophy. You are not trained in the vocational arts or given the freedom to pursue dreams and goals. You are a single human, fit to the purpose of your Lord, who enacts the Will of the Emperor. It is that will that allows you to know that the stars are full of people like you. It is His plan that allows that Empire to succeed in a dark and horrible galaxy. Your faith is your key to survival.That faith will be tested one day. It will be tested when the sky turns red and streaks of lightning begin to fall from the heavens. These streaks may turn out to be an alien invasion. It may turn out to be a sorcerous phenomena. Or it may turn out to the dimensional rifting that defines the 40k universe. A mutated human somewhere in your population just manifested psychic powers- only to have a Warp-spawn rip through to our mortal dimension from his mind! There is no warning, no defense, and no hope against these horrors alone. But the God-Emperor has provided the means for you to survive.
Enter the Imperium of Mankind. Your insignificant existance serves only to perpetuate this galaxy-spanning Empire. From His throne on the holy planet Terra does the Emperor command the mightiest military force anywhere. Your planet has a fully trained and equipped Defense Force. Your citizenship requires you, as it does all male citizens (and females as well on some planets), to serve in the DF. You are trained in basic combat doctrine, organized into local units, and given the dependable laser gun as a weapon. Every planet even maintains fighting vehicles in the form of APCs and battle tanks. But your planet is not alone.
Your planet is already barely hospitable, and it's one of the nicer planets in the Empire. Growing enough food to feed all the humans is difficult for your world, but your planet is rich in ores. There's a global industry in mining ore and the Imperial Governor (Lord of all on this planet) manages the interstellar trade for food. As long as this contract is kept, your planet can expect protection and military aid. The Imperium based massive weapons batteries on your planet and provided means for the Defense Force to fight bravely. With time, massive armies will be mobilized from other worlds and sent to your planet to aid in its defenses. Being proud of your ability to fight, you'll find the off-world armies much more competent that you'll ever be. Your limitted existance will be tested as humans of completely different cultures will tread your world, better armed and far more veteran. They'll disrespect you, they'll cajole you, and they'll discredit your world at every chance. But they'll defend your world as their own. You will find them seated next to you in the Temple on the eve before battle. They will even fight in your very house, as you would in theirs. All humans share this common goal and exercise it as mutual defense. It is truly an Empire.
The massive distances between the stars means that you'll see other humans very rarely. It takes thousands of years to travel from one star to another. But humans, ever adaptive, have found a way to conquer this as well. It is said that the Emperor created certain psykers, called Navigators, to carry His starships across the galaxy by ripping holes in the universe and driving through them. They travel through this other dimension, known as the Warp, at great peril. The creatures that live in the Warp are soul-eaters. A man cannot survive against these except through his unshakeable faith in the God-Emperor. 
If your world is lucky, it will be saved. But the Imperium is massive. War is beset upon a thousand worlds, while the Empire streches for a million. Millions of humans are lost to the ravages of war, disease, and Warp-spawn every day. But billions more are born and His vision continues. If it is not lucky, you will see war. Your planet will be reclassified as a War-World and the population will become expendable to the extreme. You will see the horror that mankind endures constantly. In the grim darkness of the 40k universe, there truly is only war!
Fate and luck are the greatest tools in the Imperium. Fate controls all and only the God-Emperor knows the results of his plan. Luck is a matter of circumstance and only the Imperium can be prepared for the horrors that await the unlucky. You, as a citizen, are beholden to both. Unfortunately, you control neither.
Good thing there's an Imperium.
Good thing there's an Imperium.
