Imagine, if you would, your brave party of adventurers enters a one hundred foot by one hundred foot square room. It appears to be empty. As your party looks across the empty space, a door on the far side opens and a minotaur in plate mail (because if you’re going to use a minotaur to intimidate a party, you might as well put him in plate mail ) walks through. While you watch in fear, the minotaur takes a step forward and taps his axe on the floor in front of him. Immediately, a ten foot by ten foot square on the floor swings down like a trap door and then swings back up. The minotaur turns to his right, taps the floor with his axe and nothing happens. He moves to the right. The warrior in your group draws his sword and charges. He only goes ten feet before a ten foot by ten foot section of the floor swings down like a trap door, dumping him in a pit of acid below. You can hear his screams through the floor. Do you understand what’s going on? Can you outwit the trap?
I used this trap on one of my gaming groups many years ago. It was actually inspired by an encounter in another DM’s game where I misinterpreted the use of a cube of force in AD&D. In any event, my gaming group never did figure out what was going on. They managed to cross the room and I think they may have dumped the minotaur in the acid himself (which is a weakness of the trap), but they never fully grasped the concept. It was an invisible maze.
Basically, as a DM I had a map of the room. On that map, the one hundred foot by one hundred foot room was represented by a drawing of a ten square by ten square room where each square represented a ten foot by ten foot square. That’s a little confusing, but it’s basically how most DM maps work. Anyway, on the map I had drawn a maze where every wall in the maze was drawn on a line separating two squares on the map. They were never drawn across the square, only on one or more of its borders. So the maze was there, but it was invisible. Whenever the plane of a “wall” in the maze was broken, that activated the trap door that dropped whatever had entered the square into the pit of acid below. I was kind with the acid. There was an exit down below so that the characters could crawl out of the acid and return back up to the maze atop. So, basically, if you crossed a wall, you fell into the acid. If you moved diagonally between two squares, you fell into the acid. The only way through the maze was to “find” the correct pathway through the walls. Like I said above, there is a flaw in this trap. Basically, when the minotaur got too close to the party, if I recall correctly, they just tapped his square and dumped the minotaur into the acid. I’m not sure if I permitted that to work or not. I don’t remember.
Anyway, the party never did figure out what the trap was or quite how it worked. They got through it, but it was largely from dumb luck, if I recall. This trap remains one of my favorite self-designed D&D traps of all time. I’m quite proud of this one.