Buy the 50 star chest from the town hall. When you open the pack, there will most likely be a key. Go down to the Trials of Gnomus place (see The Secret of Secrets trophy) and unlock the door corresponding to the color key you have. Complete the trial and take a picture of the 8 picture code you see there. After that, go to the middle portal in the floor and enter the code into the runes. There will be a boss fight section at the end of each code entered. When you finish the fight, a statue will break and you will be awarded with a new item.