Well, I have a unique anti-magic dungeon that a key NPC lich is trapped in... though, it's really only there to be anti-magical and a prison for this lich guy.

I've used the "dungeon as a prison" idea a lot actually. One time, I had a dungeon that was a series of nothing but trapped chambers linked by portals that only activated when you defeated the traps/puzzles in the room. The final portal took you to a room with a chained god in it, who defended a world-killing artifact from ever being released. The party was hired to clear out this dungeon, not knowing that they A). couldn't leave once they entered, and B). the dungeon held this artifact of world-slaying.

That campaign sadly ended before they reached the god. I was so looking forward to it too, since the god was a sad, sad fellow. He didn't have any friends, and was there since before mankind existed.