I have to admit that I hate the idea of dungeons. Maybe it’s an adult gamer thing, but I find it increasingly difficult to accept the concept of a “dungeon”. You know what I mean, an underground complex filled with monsters and traps where you can go from room to room slaughtering them. With or without a dragon at the end, I can’t suspend belief that these creatures can survive within a dungeon with concepts of feeding, cleaning (excrement), and simply not killing each other while they sit around and do nearly nothing. So having stated my bias - I know how much some gamers love the concept of a dungeon. I mean they are so easy to run - funneling the players down narrow halls, etc. So I have to justify the use of a dungeon. Here’s one:
What if the reason the dungeon has not been explored is that it collapsed? But there are all sorts of rumors, and some guy with some money determined that this site could be the location of the lost artifacts/treasure. So he brought out a crew, and they started digging - digging out the tunnel entrance. The digging crew was astonished when some of their people were killed as they were about to breech the gate.
This “lair” was a strange point of contact between the forces of evil (underground monsters, demons, stuff like that) and the surface world. I like to think of it as the fantasy “dark net”. There is something about this place that allows the evil guys to come into physical contact, and it was here that they were trading things back and forth. Now that the gateway (both the gates that were dug out and the lair itself) has been exposed, the evil guys are going to try and break out. It was some of the little ones who killed the first diggers, but now bigger, tougher guys are coming.
So the rich guy with the diggers first has to try and re-secure the gate. He’s going to start with brick and mortar, but that won’t work for very long. He’s going to need to bring in the party - super toughs who can handle these types of things. But what will they find? A smugglers’ den of the blackest type. There will be meeting rooms, store houses, and security check points. There might even be some manner of temples or chapels that some of the folks were using to communicate with dark forces “on the other side”. There will likely be some dead folks here, but they’ve probably been “affected” by the demons over the centuries. Bad guys include demons coming through as well as some undead and possibly some golem types.
But this will not be all. After they find a way to defeat the bad guys who came through, they will have to stick around for a little while so the gate can be locked up again. Yeah - evidence should show pretty quickly that the entry way didn’t just collapse but was sealed. But while exploring the lair, there may be some tunnels that don’t do what is expected. They might lead to some of the more established manor houses in the city, showing that at least their ancestors had some questionable dealings. Maybe some tunnels to the temples too. Probably some evil doers lurking within the city, and they’ve been here a long time.