I threw together a draft intro to a campaign bible to this sort of campaign.
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Blackthorn Keep - Destroyed 30 years ago along with the small town outside of it under mysterious circumstances. For months afterwords, no contact was able to be made with the keep. No one came from it, no patrols sent to find what happened ever returned.
A few months later, a single templar (identified only by his tatoo) appeared, babbling and mostly incoherent. He did say somethings about "the horde" and undead, the shadow and thralls. But nothing useful was gotten out of him and he was taken into the care of some monks and locked away for his own protection.
But within weeks of his appearance the monastery, the town where he first appeared and the surrounding countryside were all swept away by the very threat Blackthorne keep had been built to defend against. Orcs, ogres, trolls and giants crashed upon civilization like surging waves. Cities that stood for centuries burned to the ground. Kingdoms fell into chaos. Ancient noble blood lines were ended.
Eventually the horde dissipated. Thee raw hatred and greed that held it together caused it to turn on itself and it was torn apart. The remaining monsters retreated back to the north, past the ruins of Blackthorne but the exhausted remnants of the armies of humanity dared not pursue.
It is 30 years later. You find yourself on the road (or what remains of it...which is very little) to Blackthorne. You are part of an expedition, the largest attempted so far, to reclaim the keep and the town that used to support it. The expedition is lead by a man named Lord William Talmor, but is known to most of his underlings as "The Governor". Your understanding is that Talmor has the support of several very wealthy backers with an interest reclaiming the keep and discovering what happened there (or what happened to its people and treasures).
The expedition is a few hundred strong: Soldiers, landless knights, clerics, sellswords, scouts, porters, laborers, cooks, wood cutters, blacksmiths...everything a small army needs. It's first task is to clear a way to the town (Highvale...or "the Vale") that used to sit below the keep at the bottom of the pass, discover what has become of it and establish a base camp there.
How you came to be in the employ of Talmor is up to you. You can be a sellsword, a ranger to forage and scout, a cleric or templar sent to bless the undertaking or ensure that holy relics are reclaimed and lost family members of noble houses put to rest. Or you could be a scholar sent to record the history of the expedition or provide knowledge of lore surrounding the keep the town. Or maybe you are a spy hired by some other interest to keep an eye on things for some interested party. It is up you. Your salary is 5 gp per week. Your position for now is lowly...equivalent to private or apprentice or laborer. But you expect opportunities to rise as you prove yourself.
The vast majority of the expedition is human. There are a few half-elves. Halflings make up some of the laborers (often basically slaves) and camp followers who serve as entertainers. A few dwarves who have been isolated from their clans serve as laborers of various sorts and are also fairly common among the sell-swords.
Common knowledge of Blackthorne Keep and Highvale: Blackthorne existed for centuries. It was originally built out of necessity to defend against the threat of monster and barbaric human invasions. It was small at first but grew overtime into a mighty fortress. Templar Knights servered there to protect its priests and holy chapel. Landless knights (mostly second and third sons) were its leadership. Masters of Lore maintain its library. Nearby the town of Highvale grew to support it as wealth was sent from all corners of the human kingdoms to support it and its mission. The noble family charged with building and maintaining the keep (and from where it got its name) grew in prestige and wealth.
Blackthorne did its job too well. The threats it defended against slowly faded from memory. The lords of men found that sending favored sons to man its walls no longer seemed such a priority...and sending wealth that could be used to grow one's own prestige and power no longer made much sense. The keep began to fall into disrepair. Morale sunk as second sons were replaces with bastards. Lore masters began to focus on their own studies. Templars hid away their treasures to keep them safe and priests began to find it harder to commune with their patrons gods. The schemes and intrigues of rogues and spies seemed to grow to be a greater threat than monsters beyond the pass.
Then...darkness and death. The slow decay percipitated a collapse that was so fast and so complete that as far as anyone knows there was only one survivor and all its riches and arms were lost.
You are returning to find what became of it all.
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I would divide it into the a prologue and 3 chapters (maybe 4).
Prologue: The Road to Blackthorne
Survive the road to the keep
Levels 1-3
Mainquest is just to survive to the keep. Side quests might be to scout potential threats, out a spy, find rare components
Chapter 1: Highvale
Establish a basecamp in the town, secure important buildings and scout the entry to the keep
Levels 3-5
Quests would be to scout the ruins of the town. Defeat two rival monster groups (I'm thinking in homage to Return to Brookmere Endless Quest book, make the main villain a wererat). Follow leads to potential allies in the area. Find clues to what happened in the keep. Clear important sites in the town. Retrieve lost treasures or relics in the town
Chapter 2 and 3 would not be planned until well into Chapter 1. Chapter 2 would introduce the interior of the keep and some outside threats and help and cover levels 6-9.
Chapter 3 would be introduced by some radical change to the campaign (a new threat, a sudden attack that kills beloved NPCs, something) that forces the party to choose between pressing on into the dungeons below the camp or shifting focus to something else or allying with a former enemy...who knows. It would cover levels 10-12 or so.
In the end, the plan would be that the party would have established the keep or something else as their own stronghold.
What destroyed the keep and the town? I dunno...thinking it was an act of betrayal, though...because it would be boring if it was just overwhelmed by enemies.
