The characters are in a laboratory that has a locked wooden treasure chest. On the other side of the lab is an iron gate or cage that has an iron treasure chest inside.
The wooden treasure chest has a standard keyhole in the front of it. On top of the chest is a skeleton key. If the characters use the skeleton key it will unlock the chest revealing dozens of additional keys inside. Keys of all shapes, sizes, textures, and colors. Including a red key that smells vaguely like fish (red herring).
The keys inside are left up to your imagination, and what happens when they try the keys in the trunk is up to you. It could be an element type damage such as fire, acid or poison, but none of the keys from inside the trunk will unlock it.
A multiplication key, keys with playing card suits on the end, keys with a single letter or digit. The options are limitless.
The iron chest inside the cage is wrapped in a metal grate with a keyhole at every intersection. Let’s say there are 24 keyholes, but this number is not relevant to solving the puzzle.
The characters can try as many of the keys they found inside of the treasure chest as they want, but none of them will unlock the iron chest.
All of the keys in the wooden chest serve no purpose and are merely a distraction to provide confusion. Yes, including the “red herring” key that is red and smells vaguely of fish. The solution is that the iron chest is unlocked with the same skeleton key as the wooden chest.
A skeleton key is well known as being a master key that can unlock a variety of locks and things are no different with regards to the solution of this puzzle. The skeleton key used to open the wooden chest will stay in the lock until the chest is closed and can then be removed. If the characters use the skeleton key on any of the keyholes in the iron chest, the chest will unlock allowing them to access the items within.
As an alternative way to run this puzzle, perhaps the gate that encompasses the iron chest is locked and also requires a skeleton key. This could prove to make it an easier puzzle for those that realize the skeleton key is a master key right away or it could make it a more difficult puzzle for those that don’t realize the skeleton key will unlock the wooden chest, the gate, and the iron chest.
A skeleton key is a key that has been filed or cut to create one that can be used to unlock a variety of warded locks each with a different configuration of wards. This can usually be done by removing most of the center of the key, allowing it to pass by the wards without interference, operating the lock. To counteract the illicit creation of such keys, locksmiths can put wards not just in the center but on the outside as well, making the creation of a skeleton key more difficult.
: If your players struggle, allow them a DC15 Investigation check. On success, they will be able to determine that a master key might do the trick.