Ok, a brief summary of the Warhammer 40K universe for those who don’t want to go deep down the rabbit hole of the glorious Imperial history.
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We’re in the 41st millennium during which time the Imperium of Man is waging a constant, never ending, galaxy wide war against numerous threats to our galactic dominance and/or our basic survival as a species.
Aside from conventional threats (i.e. aliens that want to eat our faces) there is a powerful element of Chaos in the universe, which is a reflection & concentration of the emotions of all living beings, and Chaos is represented by several Chaos Gods which live in The Warp, which is slightly analogous to Subspace, but it allows for ships to travel within it. That said, The Warp is an exceptionally unstable and dangerous place which can destroy or corrupt ships and the minds and bodies of those within it.
The Imperium of Man - A highly xenophobic, theocratic and authoritarian body that spans literally millions of words and trillions of citizens. Their military has two main divisions, the Imperial Guard, constructed mainly of conscripts from various Imperial worlds, are largely “human” in the traditional sense and are analogues to modern soldiers with somewhat better equipment.
The other division is the Adeptus Astartes, aka Space Marines, genetically engineered supermen who posess astonishingly powerful weapons and who, themselves, are devastatingly powerful. We’re talking about guys who are 7 or 8ft tall, 500lbs of screaming muscle with specially designed organs that allow them insane levels of survivability and massively enhanced combat capabilities.
They traditionally wear powered armor that makes them, literally, walking tanks. The “leader” of the IOM is ostensibly the God-Emperor of Mankind, a super-being and psychic of unimaginable power.
Only problem is he was betrayed and mortally wounded thousands of years ago, and the only way he’s kept alive is in a gigantic device (the Golden Throne) on Earth (known as Holy Terra) which is kept running by feeding it other living psychics (conventionally known as Psykers in this universe) who are gravely mistrusted because they can be easily compromised by Chaos(more on this later).
As such, he’s not exactly an active administrator. His largest contribution now is threefold; he lights the Astronomicon, a psychic guide that allows human ships to navigate the Warp semi-safely, he gives aid to those in battle (generally by possessing them or giving them visions/strength/etc) and he’s constantly fighting the influences of the Chaos Gods of the Warp, trying to keep them from corrupting us all.
Praise be to the God Emperor, for His is the only Light in the vast Dark.
Original rundown here.
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Therefore, @Arkenbrien, creating a “demonic” character is out of the question because it is impossible in the current state of events… in a pure roleplay point of view.
Because there is already a chaotic sorcerer (can be any psychic individual, to be fair), any citizen could possibly have been subdued to his/her chaotic taint. Though with his sadistic nature, he often have a preference for trying to corrupt other psychic humans once his/her cult is going strong.
If you want a rundown of possible roleplay characters, I can help in that matter.