From the campaign:
As we all know, all D20s have a certain number of natural 20s stored in them. Once this supply is depleted, only rolling critical misses will restore them. So, how do you protect your supplies of 20s from being stolen by so called 'friends'? The Cryptex Dice Vault is your solution. With this attractive cylinder, your dice will be safe and secure!
A cryptex is a form of combination lock and safe all in one. Only when all of the reels are aligned to the correct positions can the the two cylinders be separated, giving access to the contents hidden within.