Far future grimdark setting, mankind has spread across countless worlds and are all under a central government called the Imperium. The imperium is ostensibly ruled by a ridiculously powerful Emperor, but he's 99.9% dead as the result of a civil war so his main role is as a psychic lighthouse for FTL travel, and a few different wings of government share power. In Dark Heresy, you play as acolytes of an Inquisitor, and root out heresy.
Breaking things down a bit more:
Think Star Wars tech level with gothic flavoring, and basically every race is xenophobic, especially humans. Humans only kind of know how their tech works, a lot of knowledge got lost so there's a lot of maintenance handled by ritual conducted by tech priests.
Human FTL in the setting is done through the ships travelling into and through the Warp, a demonic realm (overblown description of warp travel here: https://i.imgur.com/zHlg2f0.jpg). Psychic powers in the setting come from the warp, which means two things:
- Everyone hates psykers, because they can summon demons accidentally n' stuff.
- Everyone needs psykers to act as navigators through the warp (with the help of the Emperor's psychic lighthoue).
So the Imperium tosses them through intense conditioning to make them slightly more safe then tells them to direct ships.
Pre-imperium, back when people still knew what was going on with their tech, humanity was split into a whole bunch of nations, planets, etc, and traded and stuff. The warp was less shitty then, so they could navigate it pretty easily.
Then the eldar (space elves) started stirring the chaos pot on the other side of the galaxy, the warp became impossible to navigate using the previous methods, human psykers started appearing, and a lot of them took over whatever planet they were on, and a bunch of those either brought demons to the planet or were possessed or w/e, so it was a bad time and lasted for 5000 years. This is also part of why people don't like psykers.
Eventually a immortal psyker who was awesome in every way took over earth (now Terra) and set his sights on reuniting the rest of humanity. He was a precog and saw how shitty things were going to get so he figured all of humannity needed to be together in order to not die, and even then they'd still probably die. That's the Emperor.
He wanted to have help on the Great Crusade of uniting all of humanity, and he was the best help he knew and was incredibly tall and strong and psychic so he genengineered himself a bunch of kids using his genes as a base. Because chaos, those twenty or so kids ended up scattered across a bunch of human planets. These kids are called the primarchs.
When the primarchs got scattered, Big E took a lot of the genetic scraps left behind and grew extra organs and shit and implanted them into humans to make them genetically modified space marines. Super strong, super tall, can eat people to gain their memories, the whole package. He made one legion of these per primarch, so twenty legions.
Big E took the Space Marines went about conquering, and finding the primarchs. As he found each of them, he gave them a legion to command and told them to help out. His favorite kid, second in command was named Horus. Horus got turned to chaos and took half his brothers with him, and all of the associated legions. He was sneaky about it.
While all this was going on, the emperor was trying to tap into Eldar FTL that was less shit, basically 90% of the way into the warp, so that you wouldn't get eaten by demons, but you could tap the fishtank glass and see them. This was the Webway project. He was keeping this project a secret. One of the primarchs (Magnus) figured out Horus was betraying the emperor and went to warn the emperor through doing psyker projection stuff to go through the warp to talk to him. He found the glass fishtank walls and shattered them on his way to talk to the emperor, letting demons in. From then on, there needed to be a powerful psyker sitting on the webway project's control device, the golden throne, to make it so that demons wouldn't flood onto Terra. That was generally the emperor, he could take it.
Civil war eventually gets to Horus attacking Terra directly. The emperor hotswaps in someone else on the golden throne, fights Horus, kills Horus, and is mortally injured. His surrogate on the throne is crispy critters, he plops down 99.9% dead, and the throne keeps him on life support, he keeps the doors shut against demons and provides a lighthouse, and he doesn't really talk any more.
The loyal space marine legions split into smaller chapters, the chaos legions fuck off away from most of humanity. Add 20K years of mythologizing and you're at the present day.
The top level factions within the imperium are as follows
- Contains imperial guard (acts as standard military), space marines (acts as special forces, incredibly rare for a citizen to have seen one), the arbites (police), the custodes (make sure Big E is doing OK on Terra), and the departments of hunting, conditioning and spitting back out psykers (plus feeding some of them to Big E to keep him going).
- The emperor's mega-church. Xenos (aliens) are heresy. Demons are heresy. Psykers that haven't been broken yet are heresy. Knowing too much about any of the above are heresy.
- Tech priests who take care of all of the machinery, even though they don't always know how it works. Pray to the Ommnissiah, which everyone pretends is Big E because they need the tech priests to survive (and might be).
- This is who you're affiliated with in Dark Heresy. Basically the imperium's secret police. There's a bunch of inquisitors with incredible amounts of power (they can give commands to anyone except the emperor if they're on the case and have good cause). They're grouped into different orders that each go after a different threat to the imperium.
The big ones, the Ordos Majoris, are
- demons
- aliens
- psykers, mutants, heretics, polices other humans that seem like a Problem.
The inquisition has a pretty flat structure, with a bunch of inquisitors at the top and acolytes underneath them, with each of the inquisitors going off and doing their own thing.
In Dark Heresy, you're serving at the whim of an inquisitor, and by default, start off as very low level, disposable acolytes, barely more than a call of cthulhu power level. Players of the 40k war game make fun of the imperial guard (standard military units) as going into battle with cardboard for armor (flak armor) and flashlights for weapons (las guns). Being a guardsman is one class in Dark Heresy, with the others being even less suited to combat. Imperial guardsman make fun of the arbites (the police), and in the last dark heresy campaign I played in an arbites almost one-shot a player character.