We've all been there, looking for a way to get our BBEG or cataclysmic event into the story without feeling like we are shoehorning it into an ongoing storythread or dropping it in cold.
Sometimes a campaign will have The One Ring, but you will be forcing players to carry it somewhere and do something with it that they might not be interested in to begin with (see Matt Colville's video on Open World vs Closed Narrative for more on that).
Sometimes you start from the back and create this intricate line of events from the ultimate villain down to the players and they jump straight off the rails.
And sometimes you start with a series of loosely connected quests, trying to figure out how to connect it all together without suddenly telling them that "now you have a singular goal."
Here is my advice on how to create a world that feels alive to explore, but that has a somewhat hidden, singular force that is driving everything in one way or another.
You pick a theme, an overarching theme of something that runs through everything you do.
For instance, this latest season of Critical Role has been hunger. Everything can be connected to hunger (famine of war, the monsters in early encounters, Fjord's consuming hunger and hunger for power etc.).
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When picking a theme, you can start trying to figure out where this thing originates from (at the highest level) and what major ripples it might cause.
Game of Thrones had the Return of Magic as the major theme that is touching in everything, we don't yet know what caused this, but there is a surge of Magic in the world that caused the white walkers to finally move south, the dragons to hatch and a lot of other places of Magic to start blooming out and setting their plans and manipulations in motion.
Once you have picked your theme, it is time to figure out why something is happening on a level 15-20 encounter scale (pick a big bad early that either fits your theme or comes with a theme), you can then just hop down the line and go "how would this affect everything?"
For instance, either you can have something arriving, plotting, weaving spells, waking up or generally affecting the world around it, what happens with dragon migrations, kobolds and subterranean civilisations and creatures if a Tarrasque is waking up underneath a mountain? How does this ripple out and affect the surrounding world? You have now picked a massive, end-game level monster that needs no real reason to be waking up, but can have a huge effect on the game world.
Or, much like in Game of Thrones, what is going on might be on a divine level where the players cannot directly interfere. The Great Other and the Lord of Light seem to be somehow responsible for the rebirth of magic in the world and they are mustering their armies. This means that magical things and chosen ones and the white walkers and such are all afoot, all the while humans are caught in the middle, causing great unrest and civil war. The end-game enemy of Game of Thrones would be the Night King rather than The Great Other, but the theme is the return of magic, as is the event that causes the ripples
As such, you can have either something directly responsible for the end-game, or you can have something more esotheric.
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The Tarrasque stirs in its sleep, it hasn't woken up yet, we're not starting the campaign by blowing up a mountain and pulling up the Tarrasque from underneath (although a Tarrasque walking across the landscape would most certainly affect the world in a lot of ways, but let us save that for later, right now, it has not yet woken up, but it is getting there, the oldest creatures of the mountains have taken notice
Now we have things going on in the background as well as the foreground.
As metal prices (and tools and weapons and jewelry) are going up, with people complaining about it, maybe the players complaining about it (but getting better deals on short swords they sell etc.), the players are out dealing with crime, being hired for hunts (since there's an influx of game in the immediate area, it's where the wildlife fled to), being tasked with killing man-eating beasts that have (unbeknownst to them) been forced closer to humanity and possibly investigating increased goblinoid or gnoll raids.
From there, the flow of iron might stop completely, whilst they come across signs that the goblinoids/gnolls are fleeing from something (the kobolds and dragon) as well as any other ideas you might have surrounding how a tarrasque slowly waking up might affect the world (is there a mage in a far off tower that has some sort of alarm set to go off as this starts happening? Does he need help re-sealing the beast? Are there other things or humans trying to seek it out to hasten its awakening? etc.)
Basically, as the Tarrasque slowly stirs, all of this reaches far across the land and gets our level 1 or level 2 adventurers involved in something that normally would not have happened locally.
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What if Death is the thing? Either a lich wanting to become a god, or something nigh divine attempting to kill a god or godess of life? From this, a slow, creeping death could be spreading across everything, there are more portents, wars and duels are more lethal, sickness could spread, there is no immediate signs or hints of an apocalypse, just a general sense of "bad times", all the while, something is going on, being set in motion or starting to stir.
What happens in a city because of this? Is there perhaps a feeling of unease and nobles and guild masters and merchants all trying to buy diamonds. Not for any immediate reasons, but people are just getting a bit worried about death.
And again, all of this isn't because people realise they need diamonds because something terrible is coming, it's all just an effect from somebody important dying and maybe people just get a bit more glum and think on death more than they usually do? Maybe there are more ravens? Perhaps the god/godess responsible for shepherding the dead is absent and her clerics are getting worried, looking for portents and can give the players some quests to follow?
You then have people outside of civilised society, like a bandit Lord, also afraid of death, perhaps he has increased his raids on caravans in order to try to get his hands on a diamond for resurrection? Because he can't go into the city to buy one, and he can't trust his men to carry hundreds or thousands of gold into the city, survive, buy the diamond and want to come back.
Meaning that you could have:
After having now:
They can now start to notice or hear about other things going on in the world around them that ties back to the overall theme.
Everything is just a bit more lethal, and every one is a bit more on edge.
This also includes the wildlife:
And as the players level up and explore, you will be able to just keep building on all of this.
Not everything has to be heavy on the theme, not at all, but it all should be connected to it through some ripple effects.
Perhaps that will come back later, or the players might not ever even think to explore the forest and figure out the stream is poisoned to begin with.
But it gives you something to work with that lets you keep building and keep finding things that work with it.
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What's important in all of this is to simply look for the way something has a knock-on effect on something else, or to keep escalating when asking yourself "Why?", after all, we are setting up an escalation of events in Dungeons and Dragons (or any other RP or even a fantasy novel)
And suddenly you have a multi-step adventure:
And this brings me to my second point in these closing words
Just because you do not want to run an open world campaign with some massive, held-together theme does not mean you can't use this method to craft quests, build villages or link character relationships together.
This entire article is, after all, about ideas and methods for you to take and apply to your own game as you see fit.
Just remember that even in a big, thematic open world adventure where it eventually leads up to something cataclysmical or huge, not everything has to be directly connected through trackable actions (the dead aboleth could well have just died on its own), but rather through the overall theme and effect said theme has on the world.
It is completely okay to have individual storylines that do not all lead to fighting the BBEG or are immediately traceable to it, as long as you are aware of how to reconnect your players to the overall story after they complete such a quest-chain.
It is part of the beauty of what makes an open world open.
And after all, it is your adventure to run.
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My background: I might be somewhat newer at DM:ing than a lot of people here, not having started proper RPG's until my early twenties (I am now 29), however, I've been doing a lot of it, whilst also spending the past eight years of my life either studying writing or writing novels. I am not a perfect person when it comes to rules, luckily, I have great players helping me with that. My focus has always been (and probably will always be) on the world and stories that form within it.
If you, as a DM, wants to use any of the above scenarios, themes or plot-hooks for your world, feel free to do so, and I trust that you will make an amazing adventure out of it.