I just had a great campaign idea. A BBEG sets up a bunch of small dungeons with “keys” that when put together will access his main base for the party to break into.
The twist is these “keys” have clues that just lead to other “keys” and dungeons. The BBEG doesn’t have a lair but is trying to rid the empire/continent of their best heroes so it’s easier to take over.
There can be clues for the party like groups of dead adventurers in the dungeon or an easy one like a “key” just leads back to the start of the dungeon loop again.
I got this idea after seeing a lemmy post about ant death circles.
I like it, because it can also work as a conspiracy. The initial quest giver can be an evil lieutenant posing as an ally. The loop can be hinted at by clues in the dungeons themselves. Maybe there are errors left behind that hint at a dungeon/s they’ve already finished. I could see a lot of ways to mess with the PCs.