Here is some of the history of my megadungeon, Felltower.
A bit of this has been summarized before way back in 2011. This is all stuff my players have heard piecemeal over the past couple of years of play.
The dungeon complex known as Felltower has been under continuous occupation by someone or something as far back as records can be found. Occasional comments from extremely long-lived races, like the elves, say that the records just expose the tip of the iceberg. They also hint that some things have been there for a very long time, and that even such as the elves are newcomers compared to things deep below the mountaintop.
From what is known to humanity and their allies, the area around Felltower Mountain (its old name is long lost) has always been dangerous. It has always been home to orcs, trolls, and even dragons. The orcs call the place Grak Yorl, which means something like "the Boneyards" or "the Black Fortress"* and claim it as some kind of ancestral holding. What the orcs mean by "the Boneyards" and what they want with the place isn't discussed; the average orc doesn't seem to know or care beyond claiming ownership.
The first major human occupation in memory was a group called the Black Brotherhood. The Brotherhood consisted of wizards, priests, or wizard-priests (the records vary) who dug deeply into forbidden and dangerous magical experiments. There are hints that they came to the area around the mountain looking for something. There are also hints that they set up in order to tunnel down to some forbidden or secret location deep down. In any case, their enemies (the Church of the Good God, assorted wizards, all good and decent people, possibly even the orcs - although the elves and goblins say that's a lie) eventually marched into the wilderness around Felltower, sacked and burned the Black Brotherhood's fortress, and put everyone to the sword. And burned whatever they could, before sealing the place up as best they could. Hints from this time say that the Brotherhood expanded upon tunnels and caves they found below the mountaintop.
In the centuries between the area fell back into the hands of assorted smaller groups. Periodically the residents would change, temporary visitors would come following up on some rumors of treasure or magic or whatever. Others came for even more obscure purposes. But no large group really seemed to come and stick, as far as humanity knows.
Centuries later came human occupation. First a little trickle of adventurous sorts, but later came an organized military force. Baron Sterick the Red and his followers, all largely northern barbarians, occupied the upper levels of the dungeons. Sterick caused a castle to be built on top of the mountain. He and his followers waged battles around the area, trying to build up a kingdom on the surface. They turned their best resupply point into a city (still called Stericksburg.) They also took lots of prisoners, and put them into the dungeon complex below. They also sacked some rich neighbors, human and dwarven and otherwise, and hauled their gold back to the castle. The castle was later ruined when his nascent independent barony was put down by his former liege and an alliance of enemies and opportunists. Rumors abound that Sterick's most loyal followers, his former prisoners, and his treasure were never brought back up from the dungeons. What happened to Sterick was a secret held by the most senior of the alliance. Much was recovered during the sacking, but the rumors of more of all of them persist. Dating back to this time is the edict (still valid) that says that anything done under Felltower is legal, and anything taken there is the property of the keeper. At the same time, the ruins were declared legally off-limits, subject to severe punishment, and that a curse had been laid upon them. It's not clear if anyone has ever been punished for it.
After Sterick, there has been no civilized occupation of the ruins, but recently organized groups of cone-hatted cultists, hobgoblins, and now orcs have taken up residence. Over the years crazed loners, evil wizards**, and bizarre cult members have trekked up to the ruins. Very few ever came back to civilization. More recently folks just started to refer to "the ruins" or "Felltower" but everyone regarded people who went their as fools. The few who returned came back broke, some that didn't were found dead, and most just disappeared.
The present day has seen a growing renaissance in Stericksburg, ever since a group of adventurers started plundering Felltower's ruins. More outsiders have come to Felltower, too, and set up occupation in it or above it (such as the orcs), too.
* Translating orcish is difficult.
** Are there any other kind? Dryst says no.
And this is known, and possibly related or unrelated to the occupations of Felltower:
Over this whole time, there have been rumors of dragons in the area. An oft-repeated tale says there is a gigantic dragon that sleeps under Felltower, and dragons have been spotted flying north of Stericksburg . . . although not for some time. A smaller dragon has been spotted to the north, by hunters and scouts, but it has always flown south toward Felltower before disappearing from view. How or why a dragon would be in close proximity to human occupation is not known.
The above is mostly true, probably, and mostly reliable. It's subject to revision by events . . . not all that is known is true, and not all that is true is known. There is a lot of conflated information, lies, and just honest mistakes mixed in to what is known about Felltower. But out of game, I will say that it all provides some value and context to the seeming nonsense of things going on in the dungeon.