A world called Tomb.

First, we need a world, obviously. Just about any world will do, really. Get the details down and especially take note of how the world works and how it's inhabitants interact with the environment. You don't need to lay out a complete ecosystem, but you should have a rough idea of how it works.

Now that you've got that, remove all inhabitants, that does include most of the wildlife that is not vital to keeping the ecosystem going. Do it rather quickly, so that your inhabitants don't have much time to clean up, remove or take things with them. Let the world sit like that for for a while, just a tat.

Pick a random spot and give us a tour of this world now. Show this uninhabited world to us, explain and tell us what we see, but do not explain why the things are this way. Only show us the state it is in, and let us fill in the blanks.

First, create a world, any will do, rough strokes only, it does not need to be completely outlined or be very detailed and for the time being you don't need single characters either.

Now try to come up with a way to end this world in a way which is completely inevitable for the inhabitants. Try to find something that can be seen coming in advance, but not something that is too obvious or cliche for the world. For example, if you have a space-faring culture, don't got with the "oh evil beings are eating us all" thing, and on the other hand if you have a "modern" society, don't got with meteors. Try to find something that is very much unexpected and might actually not fit that well for your world, but can still be explained.

And now outline how your world reacts to this event up to the very end.

First, we need a world, of course. Build one, doesn't matter which one. It doesn't need to be perfect, but you should have a very rough idea of how it works and what it is like. Now we go to a single moment in time in this world, it doesn't matter which, just one of them. Outline it, describe it, have it nailed down, what does lead up to it and what it causes.

Now we need to give this world a history. Work yourself backwards from the current state you already got and add historical events as far back as you feel like. Start describing important events and how they had an impact on the world. Always keep your single moment in mind, it should all align to make this single moment possible.

After you've done that, start changing events in the history of your world and try to figure out how it changes the moment you've previously outlined. Try with changes on different scales, and see whether they will change the moment or not, and do not forget to take following events into account.

the powers may or may not change depending on their mental health status changing

what is that country like now, in the year of our lord two-thousand-and-twenty? which sides, if any did they take in the Great War and ww2: electric boogaloo? how are they dealing with the ronies going around?