The Hidden Den is a secret location hidden throughout Rakenzarn. Each region contains a Hidden Den where it can only be accessed by inputting a password that you have defeated from a bonus boss of the previous version.
Each Hidden Den contains 3 crystals with powerful equipment. In order to open the crystal, you must bring a party member or a supporter member (who must be equipped on Kyuu only) to open it. Each crystal has a symbol that hints as to the recruit you must bring.