Ruin Key

The Ruin Key is a Rare item in Golden Sun: The Lost Age. It is found in Madra Catacombs and is also used there. It can only be reached if you have the Frost Psynergy, meaning Piers must have joined your party. It is used on a big red door to open it up, and you will gain the Moloch summon sequence. It is located in a chest on top of bookshelf and can only be retrieved with Tremor, which causes to fall into the room below where it can be reached.