The natural approach for me has been to bounce back and forth between recent and ancient history. The 2E Worldbuilder's Guidebook has some good random tables. My general pattern has been to stick to the immediately relevant details during the session, and then jump back to fill out origin stories for most of the elements when I have the time. The middle history of something tends to get filled in through the process of coming up with origin stories for other things.
Let me mash out an example of my process.
You might start with a random dungeon. The most interesting feature might be, say, a magic pool that grants a one-time permanent +1 to a random physical ability score and 2d10 years of rejuvenation. We can dig up the old Sumerian dictionary for a good name later, but for now it's the Well of Dwayne.
The Well of Dwayne is an outflow of natural magic, possibly the original reason for selecting the site of this structure.
So far, we have a few things:
The Well of Dwayne: Technically, the well itself was constructed as part of Dwayne's Keep. The water source, Dwayne's Spring, has always been where it was, or formed during the last magical cataclysm. For our purposes, we can treat the date it was discovered as its origin. Discovered 680 years ago. All years will be rounded to the decade. Roll a d10 and add it on if you don't like it.
Dwayne: Just by assigning a name to the Well of Dwayne, we've generated a historical character. Presumably, Dwayne discovered the Well.
Dwayne's Keep: Dwayne himself didn't necessarily have anything to do with building the Keep. Technically it's named after the Spring. Built 600 years ago, apparently to protect the Spring from something.
So, sometime about 680 years ago, the swordmaster Dwayne discovered a magic spring that made him stronger and restored his youth. He began sharing the secret of the spring with his students but to his dismay, someone overheard them talking in town, and soon dozens, then hundreds of pilgrims began coming. One day, someone was murdered and their blood spilled into the spring, and everyone there found the waters to be cursed, weakening and aging those who bathed in it. Decades later, the Dwaynian Order agreed that the cursed spring needed to be sealed off, but remained hopeful that the curse would eventually fade. One of the Dwaynians volunteered to test the water, and it seemed that the blessing had returned. However, the town was not convinced.
First pass timeline
680 YA - Dwayne discovers Dwayne's Spring
670 YA - Dwayne's Spring defiled
600 YA - Dwayne's Keep built, diverting the Spring into the Well of Dwayne.
This story creates a number of additional hooks. We've defined Dwayne as a 'swordmaster' but possibly also someone who studies natural magical phenomena. Most likely a ranger. The Dwaynian Order might be a society of rangers. There are other class possibilities, but this works. But now we might look at what caused the Dwaynians to abandon the Keep.
Maybe they went extinct. Maybe they had to move. Maybe they moved and then went extinct.
The Dwaynian Exile: A good tip to remember, is not to make too many assumptions about how something happened before considering the options. All that definitely needs to have happened, is for the Dwaynians to have lost control of Dwayne's Keep, because, remember, it's supposed to be our dungeon. Maybe the Dwaynians decided not to make it widely known that the spring was clean again, so the townsfolk believed it was still cursed and possibly emitting some form of evil influence. The debate over whether it was more important to protect the Well of Dwayne than to be trusted by the locals probably split the Order. Those who remained were a ready scapegoat, possibly for someone deliberately planning to commit a crime and let them take the blame. Or, just some unfortunate coincidence provoked the townsfolk to get out the pitchforks. A curious child wants to see if the Dwaynians are so bad, has a fatal accident or encounter with wild animals on the way, and never makes it home.
The Calendar: Then there's the dates. What is the current year, counted from what? This seems like such a small thing, but it's a major springboard for delving into ancient history. Having a certain number for the current year suggests something important happened that many years ago. That's a whole other exercise. It's enough to say that the current year is 3730 of the Dozenth Epoch.
The New Protectors: Let's say the Dwaynians did go into exile. If they were protecting the town from something, the town would then be vulnerable to, roaming owlbears or whatever. That doesn't mean they'd be helpless all of a sudden; someone else might be ready to fill in the gap. So, maybe the town survived with a different source of protection. They might have had to accept an oppressive deal, costing tribute or extra taxes to post guards. Or a different group of locals rises to the task, possibly themed around a different class. Most likely we don't want full spellcasting types, because they'd be able to investigate the supposedly cursed well and disprove the rumors too quickly. The new protectors should be far more defensively oriented, and not be well equipped to disprove the rumors.
The Turtle Strategy: I'm inclined to go with the tax hike and extra troops from the local ruler. The town walls are built up, the guards have enough outdoor skills to be capable of short hunting expeditions as needed, but they live in town and don't like to go on long scouting treks. So, whatever is out there is not being closely monitored. But the town itself is reasonably safe, and might still be able to keep growing, presumably in the opposite direction of Dwayne's Keep. That growth might be one of the things that eventually causes things to change-- at some point, they can support the presence of a leveled cleric, bard, wizard, whatever.
There's still a huge chunk of middle history to cover yet, and we need to do something with the Dwaynians in exile. Now, the Dwaynian Order was never supposed to be a High and Mighty Ancient Society of Sworn Protectors, with Secret Handshakes and Solemn Oaths. Just a small ranger's guild that got kicked out over a misunderstanding while trying to protect a unique natural wonder. Many of them probably moved on. A few might have tried to keep watch over the town and Dwayne's Keep, and try to get the truth out there somehow. It's time to shift gears and work backwards from the present.
The Quest: The big catalyst in the present is that a real bard has moved in to the town which still needs a name. She's interested in the legend of Dwayne and has been trying to get everyone's oral version. Over the years, the Dwaynians' descendants have managed to plant some of the true details into a few people's versions of the story, but the result is a contradictory mess that the bard is having a lot of fun untangling. She wants to commission an expedition to Dwayne's Keep to see if anything there that can help establish the true story, but everyone who lives in the town is too afraid to go.
The Real Threat: Whatever is keeping people away from the Keep, it can't be purely fictional rumors, because some curious kids would have exposed it by now, so something must have taken up residence relatively soon after the Dwaynians' exile. It has to be something dangerous enough to give trained soldiers pause, so in addition to the owlbears prowling the surrounding woods, I'm inclined to say that there are also trolls. That is to say, the guards might still have fought them if they approached town, but generally not not gone hunting for them.
The trolls might even have another lair somewhere away from Dwayne's Keep, which explains how they might keep returning while the town fixates on the Keep as the source of all nasty things. One or two big raids might help fill out the middle history.
Second pass timeline:
3050 (E12) - Dwayne discovers Dwayne's Spring
3060 - Dwayne's Spring defiled
3110 - Dwayne dies, his disciples continue doing business as the Dwaynian Order.
3130 - Dwayne's Keep built, diverting the Spring into the Well of Dwayne.
3140 - After many Dwaynians abandon the Keep, the town of Threshington gets all pitchforky over the disappearance of little Timmy and runs the remaining Dwaynians out of the area.
3160 - Threshington builds a town wall, and makes a deal with Duke Jeff to post a few troops there, for extra taxes. The area around Dwayne's Keep is shunned by the townsfolk.
3190 - Threshington guards record the first reported sighting of trolls in the Dwayne's Keep area.
3220 - Threshington's incendiary defensive siege weapons are an effective deterrent against troll raids, but the town remains poorly equipped to mount an attack on the troll lairs. Guard Captain Bob's disastrous attempt at a counterattack is recorded as Bob's Folly.
3580 - The Battle of Threshington, where a band of trolls attacked during a rainstorm, and had to be lured into the town hall which was set on fire.
3730 (Present) - The new bard is researching the legend of Dwayne, and wants to commission an expedition to Dwayne's Keep to help with research.
And so on. The next pass, if there needs to be one, might look deeper at who Duke Jeff is, how the town defenses were upgraded after Bob's Folly and the Battle of Threshington, the second troll lair, and the identity of the pro-Dwaynian storytellers.
To make larger leaps back in time, you might go to the founding of Threshington, the Duchy of Jeff, and the kingdom of Thisland, although that probably only extends back some number of centuries. Connecting this to ancient history, though, we might decide that we want to maintain a wild frontier feel in places like Threshington, which gives some shape to what needs to happen to explain why that region isn't very settled.
I hope this was informative, or at least amusing.
Dwayne.
