Note: This is a puzzle from a recent adventure. It can work anytime you have a decent level wizard in the area to create a magical barrier of acid(use your imagination).
This appears to be nothing more than a doorway. There are no traps on the door, and it is not locked. If the characters are specifically checking the FLOOR for traps etc, they notice a fine layer of dust on the floor, but nothing else. They do not find anything unusual on or around the door.
When the characters open the door, a loud whoosh is heard and the doorway appears to be covered by a shimmering, liquid layer. It has the distinct smell of acid to it. A magic mouth appears on the door that has just been opened and it speaks.
iYou’ve probably already noticed that the doorway has turned to a magical acid barrier. The acid will instantly destroy any non-living material passed through, and will disintegrate any living material larger than one arm. Feel free to try it out if you like.
On the floor, on the other side of that barrier is a loose stone in the floor. Under that stone are two metal bars. One of the bars is magnetized. Hanging on a string past the barrier, just out of reach, you’ll see a metal ring.
If you put the non-magnetized bar within one foot of the ring, the acid will strengthen and eat off your arm. Unfortunately, the effect of the magnetized bar extends only six inches. So, you have to KNOW, before you put a bar up to the ring, which one is magnetized. If you put the magnetized bar up to the ring, it will remove the barrier. If you put the non-magnetized bar up to the ring, it will melt your arm off.
Good luck in figuring this one out. Judging from the dust on the ground outside, you’ll need it./i
The barrier destroys any non-living material passed through it, including magic items. Living material feels a tingling but is unharmed.
Any attempt to grab the ring by characters results in it moving away. It cannot be grabbed or removed from the string by hand.
The solution is that the magnetized bar will pick up the other bar when placed at the very center. The non-magnetized bar will not attract the magnetized bar if it is placed directly in the center. Obviously, both bars will attract each other if the ENDS are used to attract, so the key is using the centers of the bars(where the magnetic field is neutral) to find the non-magnetized bar.
If the non-magnetized bar is placed within 6 inches of the ring, the acid barrier strengthens to eat off the PC’s arm. Once they capture the metal ring with the magnetized bar, the barrier disappears.