The “dungeon” is quite legitimate. It is the former lair of (Insert dark and dangerous local menace from your world). You are sold a map of some antiquity and it is genuine. The location was on the outskirts of civilization generations ago. It is also the last known location of an important item.
Today, there is a good sized village not far from there in the lowland. There are similar villages all around this region, as a religious group has move out here to find some peace. There has been (at least) two generations of kids from the village that have roamed the hills here. They found the opening long ago. Some kids kept it secret, others brought their friends. You see, the people in these parts belong to a strict sect. The freedom they could find here was important to them.
The kids spend time alone here. They sing and dance (something prohibited in the village). They have parties. They couple. Some practice with small slings against the rats. They do all sorts of things in this set of halls.
Now those kids are parents (and grand parents) themselves. The halls (as the locals call them) are an open secret… something that everyone conviently forgets when they become an adult.
Now if someone who is skilled in woodcraft can tell you, there is almost a trail made leading to this place. (Of course the map takes you up this torturous course. So if you encounter kids here, they will go, “why did you come up that way, it is easier *jerks the thumb* that way”.)
There is a small warren of cave walls and carved hallways here, about 10 rooms and 18 hallways in all. There is trash and the remains of the kids who have been here in these wall. Any traps that were here have long been triggered and broken by the kids.
Now most adventurers might just turn around. Their mighty dungeon has been looted and invaded by kids.
However, in the back of the halls, there is a chasm - A deep and dark and dangerous chasm. The hall that leads to it has been blocked off by a couple of up ended tables long ago. If the adventures follow this route, they will find the rest of the Lair of “Dark and Dangerous Menace” with the rest of its traps and trick, and the artifact/ item they seek.