Easy. The Labyrinth of Madness, or whatever its official title was, which featured in a recent campaign in a D&D mod that my friend Rich created. It was a subtly terrifying, harrowing experience that my character at the time, a deaf Aasimar cleric, found deeply troubling.
After passing through the many strange halls of ancient gods, and battling across the altar room, we rescued the damsel in distress and tried to find the exit by following some steps down behind the altar into a series of narrow passageways. We were confronted with a giant stone face, looking sternly at us. Written above it in a language I barely managed to translate was the phrase “Wear my crown to win your freedom;” or something like that. Confused, we continued on. There was a side room filled with coffins, chains, and a treasure chest, but that’s really a separate puzzle from this and proved far more difficult than it needed to be. Anyway, we walked through numerous passages, exploring the small complex we had discovered. There were occasional traps, so we moved slowly with regular spot checks, but the only other thing of note that we found were a few more mad words in a dead language and a sealed room. When we broke it open, we found an angel which had been trapped there for years. Locked in a room for so long, it had gone completely mad. There were long gouges in the walls where it had scratched at them with its fingers. Though it had once been a being of good, it was now so twisted from insanity that it attacked us. My part-celestial cleric prayed through the entire encounter and did not attack, unwilling to strike such a creature. It was tough, but everyone managed.
Anyway, that was about when we found out the real trick of the maze. As we kept wandering, we suddenly began to realize that we’d seen some passages before. As players, we could see that as we stepped off the map in one direction, we would enter it from the other side. The maze was looped. We could run through the tunnels forever and only ever find the same few corridors. That was when we suddenly became a little worried about this place. What really completed the freaking out was when we found the stairs down, which we gratefully took to find ourselves at the start of the maze, coming in the entrance. The stone face was now twisted in maniacal laughter. We were well and truly trapped.
A few of the party turned and went back up the stairs, using classic puzzle logic. They wound up instead in a small room with a few corpses of people who had been trapped there for sometime. As they inspected the room, the trapdoor shut and vanished, leaving them sealed in an extra-dimensional place, potentially for eternity. We got them out later, just by going up the stairs ourselves and holding the door open, but it was a very dangerous little side room in theory. I studied the writing above the face again, however I found that by staring at the face for so long, strange thoughts entered my mind. Essentially, the face drove me mad. I cackled and rolled around the floor in laughter. One of the others was caught by this until someone put a cloak over the face.The trick, of course, was that while I was mad, I knew the way out. Once the party got me thinking about the exit, I ran up the stairs, which suddenly continued onwards. The maze broke, the face became enraged, weeping blood which rapidly filled the labyrinth. The party fortunately decided to follow the mad, running cleric and we all escaped. The beauty of the labyrinth was in the details I think. We knew from the start there was something about insanity, and there were clues that suggested that the longer you stayed in the maze, the more inevitably insanity became. This wasn’t an issue until the maze was revealed to be looped in all directions. When we walked down those stairs to find the face laughing at us, everyone got really freaked out. It was awesome. Best trap ever!