- I don't want my underground complex to be of Dwarven or Skaven make.
- I want the front door to lead to a heretofore unknown valley.
- I have some Halfling ideas rolling around in my head.
The resulting background is:
Many ages ago, the valley was home to Halflings. One family of Halflings began dabbling in magic and eventually moved up to a tall hill overlooking the valley where they could practice their art without endangering, and with less interruption from, others. The power of this family grew until it was summoning demons to do its manual labor, including excavation and artisan work in the hill complex.
The complex will be like a Hobbit hole but with expansions. One expansion is the family crypt, a sublevel of the complex. Occasionally, the family would allow a particularly prominent (and giving) Halfling from the valley to be buried within.
The ages saw the decline of the population in the valley, the loss of its connections to the rest of the Old World, and the extermination of the family in the hill. Weather all this was natural or unnatural remains to be discovered by the adventurers.