The Treasure Chest Key is an item found in both Dark Cloud and Dark Cloud 2. Its in-game description reads : "A key for a treasure chest. Defeats traps, too." This item, as its name implies, is used to unlock locked treasure chests encountered in Dungeons (not the chests where you have to disarm the trap by guessing). Using the key will allow you to open locked chests without risk, avoiding the trap that would have been set if you forced it in order to open it. This item is quite rare, when available from Donny it's expensive, and it'll become inventable when you reach the Chapter 7.