The initial idea for Harrow Home Manor is here: Harrow Home Manor. I've been casting about as I consider various ideas for an early modern, horror/adventure setting for D&D, and Harrow Home Manor was the latest.
I'm seeing a great opportunity in placing a site where there's a lot of history, and certainly the British Isles qualifies as rife with it. Deep beneath the Yorkshire moors, I'll place a sleeping, pre-Celtic god-monster, the inspiration for monstrous myth figures like Balor or Crom Cruach from Irish Myth. The monster has awoken at different times to bring death and madness to the surrounding lands, and grant obscene powers to its scant worshippers.
Conflict against otherworldly forces has raged on the site time and again, such as when the Pictish worshipers of the beast were driven off by the Celts; the site was sacred to the druids, and then taken from them by the Romans. A church was placed there for a time, following the early Dark Ages practice of coopting pagan sites with Christian chapels, but the scant population didn't support a church on the remote heath and the land was claimed by a sorcerer in the late Medieval period after the church fell into abandonment.
Taking a cue from "The Case of Charles Dexter Ward", a distant relative from a southern city inherited the property in the 16th century, restored the fallen manse, and began exploring the dungeons and crypts beneath the ruins. Discovering a cache of sorcerous lore, the descendant restored one of the Medieval wizards to life and the great project was resumed.
As mentioned in the earlier Harrow Home post, the restored wizard set about gathering from the corners of Europe those few bearers of forbidden knowledge that had thus far escaped the fires of the Inquisition, bringing together collaborators on the great project, to awaken the sleeping god and restore the world to the madness of a bygone age. The lure of power corrupts, and the gathered host factionalized and fell to destructive infighting once it was clear the project would take extended effort to recover the lost secrets of waking the sleeping god. Less than a century later, Harrow Home is again a crumbling ruin with an evil reputation.
The part that's interesting to me about this mega dungeon approach is how it can be made "character driven". I can develop portraits of these powerful, insane, gothic figures, and populate the tombs, crypts, caverns, and depths with the fruits of their mad schemes and revenge plots. There's the sorcerer, burned at the stake or beheaded and returned to a mere semblance of life through undeath, who uses mortal pawns to kidnap the descendants of the handful of Medieval knights that ended his existence the first time, 200 years earlier. His revenge plot spans centuries.
There's the Carpathian blood-drinker, bored with life in the dreary caverns beneath Harrow Home, who has taken over a local abbey and changed the rites and rituals to fit a religion more to his tastes. One hazard in the dungeon is the body parts of the severed sorceror, the crawling hands and arms that scurry around the dungeon looking for the sorceror's head. When the wizard proved too difficult to kill, his enemies had him dismembered and scattered. His head lies in a trunk, thrown into the bottom of a well. He's quite angry.
And so on. Lots of mad, bloody wizards, lots of fun little stories.