First Edition AD&D-style dungeon crawl with more modern rules. It’s not going to be easy, and with a first sentence like that one, you shouldn’t expect it to be. Here’s the basic premise, without violating any applicable copyright laws:
A long time ago, there was a temple to one of the lords of the Abyss in a city of vast evil. The armies of good united to try and destroy this temple. It worked. The evil high priests fled their ruined temple and city, pursued by the white hat-wearing dudes. No one knows what really happened to them, since neither they nor the army of paladins, clerics, and other do-gooders was ever seen again.
The forces of evil aren’t that easily thwarted, however. As the proverb goes, “Evil will always triumph, because good is dumb.” These evil jerkwads found a place to hide, and kept doing what evil always does- I don’t know, kicking kittens and stealing candy from old ladies trying to cross the street, or something. No one knew they were there, because, duh, evil = sneaky.
So, they were working on a new temple, this time underground. After a less long time than a long time ago, they were finished. Not content with living in a nondescript hole in the ground, they decided, “Hey, let’s build a totally inconspicuous mausoleum and graveyard on top of this hill. No one will ever come and check it out.” After this last phase of construction was completed, the people in the villages nearby noticed that the woods were more “Evil Dead 2” and less “Ferngully”, but they never figured out why, because they were villagers, and if villagers were at all heroic there would be no need for PCs.
More recently, a group of adventurers led by a high priest of (insert favorite good deity here) decided they preferred “Ferngully” forest over “Evil Dead 2” forest and rightly deduced that maybe that strange mausoleum on top of that hill had something to do with it. They explored the area, delving deeper and deeper into the stygian depths (had to throw some Lovecraft vocabulary in here somewhere), and found a plethora of evil creatures. Some of the party survived; the high priest was never seen again.
For the last hundred years or so, numerous groups of adventurers have ventured forth. Many have died in the wilderness, because this is heavily influenced by First Edition and the wilderness WILL frigging kill you. The rumors, spread by those who somehow returned, hint that even more fell to mysterious guardians of green stone or were slain while still above ground. “All who have explored Rappan Athuk offer this one universal piece of advice- ’Don’t go down the well.’”