Building a world is a hard thing. Very hard. It's easy to look out your window and describe a world that way with whatever improvements you need to give it (advance it enough for sci-fi or the reverse for fantasy) and build from there. At least you'll have something to build off of as far as I'm concerned. But how does one describe a world that's fallen into corruption and decay? No, not the political decline or social decay. No, I'm talking of the pure science fiction type in that of good/evil, angels/demons styled decay and corruption? Imagine a world in which it starts like a futuristic version of what is Earth now, but one in which they are subverted, perverted, corrupted, mutated, and countless things that take it from what was the far off Earth type futuristic world, to one that's so far down the path of (not completely, but enough for you to visualize it for me) a "Hell on Earth" type setting. Not really the "Fire & Brimstone" styled description that we're used to, but still in theme of that kind of damnation. Thoughts?
As with any speculative fiction, you go with cause and effect. So if A happened, how did it affect this and that, and how do these effects affect other things. And you just keep going till you answered all the cause and effects.
Never really thought about it, but novel #3 of my planned trilogy is on a different planet, so this is a good exercise to begin world construction. How about: start with your senses. Sight, sound, taste, smell, touch. Describe / consider the scene from the perspective of each of the senses. Choose the one that is most different to "normal" and use it to show the difference. Choose the one that is still most "normal" and use it to show the similarity. Similarity allows the reader to empathise or connect with the familiar, then the stark contrast is like the barb in the tail proving this world is in fact alien to their understanding and experience. Caveat: never done it, just make stuff up as I going along, could be full of baloney.
Well, the better you understand your world, the easier it is to describe and imagine. Such as: if a tear in the Earth's crust released a dangerous gas, how would it change the world as we know it? Well, the gas firstly kills most flora as it settles around the tear and mutates the fauna heavily. The surrounding towns are abandoned causing many refugees and illegal immigration. A military research has been erected there to both contain and research the spread of the gas and the mutated wild life. Fire was used to destroy the surrounding areas to prevent the mutation from spreading and walls to contain the gas. It was too late too contain, and the walls cannot stop avians from getting by so there's some issues there. Kids dare one another to see how close they get to the boundaries. Occasionally, someone gets lost in the gas and gets infected. They mutate and are put down or hide for safety and become hunted. Nation wide security measures are being put in place to contain any new and sudden tears that would release more gas causing unrest and even paranoia. And so on. You just keep thinking how this dangerous gas would affect people.
I'd give some inner workings to my mindset if I had posting rights in the works section here, but am still lacking in the criteria dept, so I'll have to go a little further in the description I guess, eh? Imagine a world in which the boundaries between the mortal plane and what would constitute a demonic plane of chaos would exist. It's a clear and physical separation between the two, but much akin to the similarities between heaven/hell/earth, our strings in the mortal plane can be pulled like a puppet and a puppeteer. In the beginning, it's subtle as their abilities to reach across the nether between realms is initially a fairly weak and tenuous thing (at best). But over time, as conditions are met to strengthen their presence in Point A (aka the mortal realm of what you and I call "Future Earth") and Point B (aka what you'd call the realm of Chaos), their influence grows accordingly. The entities of this realm of Chaos aren't just your normal daemons, either. They'll range from their gibbering minions of the stereotypical near mindless daemonic entities bent on wanton wholesale slaughter, hacking, chopping, biting, ect anything and everything, all the way to far, far more prolific daemonic beings of immense power and style. In general, their modus operandi will vary from that, to all the forms of plague & disease, all the way to sensory stimulation (aka sins of the flesh on steroids, tripping on acid, cranked to 11 and on fire. All chased by Ninjas. Why? Because it seemed like fun to them at the time) The world shaping to their presence also needs to be reflective as well, it just can be "Earth Part 2 with weirdos running around" in its general look and feel (we have that already on earth, it's called New Orleans during Mardi Gras).
The only worldbuilding advice I'll put in is that you DON'T need to know everything about the world and it's history. You need to know enough of what the characters see to make it feel real. If your story is a play, your setting is the stage ... and those buildings lining the street scene at the back of the stage are usually flat slabs of plywood painted to look like buildings. Most of your worldbuilding is going to be plywood flats, not huge buildings, you only need a three dimensional, properly constructed thing when it affects your characters directly. The farther away something is from your action, the less detail you need. Which isn't to say the far away stuff isn't important - my project is set 20 years in the future, and I know a lot of what happened in the intervening 20 years because it matters to what's going on in the year 2034. I know who "won" the next five U.S. Presidential elections and roughly what they did to create the world that my future news reporters are covering. I also know who the next four popes are because my story spends time in the Vatican, and I know a bit about British politics because I reference an independent Scotland. Do I know the full composition of the U.S. Senate or the name of every Catholic cardinal? Nope. And I'm probably never going to. Because it's not important, and it's never going to be important unless I accidentally reference too many senators from one state (now that I think about it, I should have a list of politicians I reference, just for consistency). But you get the idea. Know what you need, not everything.
You need to describe the aspects of the world that the characters interact with directly. Having done that, there is a danger the reader will be bewildered by these features and wonder where they came form and how they are sustained. The reader may even lose the plot entirely. Giving some back-story and explanation would be helpful. This may need to be done before the feature is presented. If a demon is suddenly introduced, the reader may think 'What's going on now?' and be left confused until the explanation is given. Alternatively, if you have the characters talking about demons and then later in the story a demon is introduced, the reader would think, 'Ah yes, I know what this is.' Try to avoid info dumps. Spread the explanations out and use a variety of different methods to present it. I'd start with the story and work out from there. I'm not perceiving sensory stimulation as a particularly bad thing. I have tripped on LSD and either you trip or you don't (there is no 11) I'd describe the experience as 'freely imaginative'. The idea of mindless people engaging in wholesale slaughter, hacking, chopping etc, sounds a lot like IS to me. All you need do is make 'faith' a dominant influence, and all hell breaks loose, as it has done in the middle east. I doubt you'd actually need to reference other 'planes', there is enough potential within people already. It's those who believe they are doing the greatest good, and building a new and better world, who commit the greatest atrocities.
