First you go to The Crystal Catacombs to enter the Elder Vein Mines (don't forget your Mining Helmet), there you must collect all three pieces of the Corrupt Crystal Dampener. After you have found them, make your way through all the mines again to get to the Omega Beet Mine, where you will find the Corrupt Crystal Core. When you enter this mine, go to your right and pull the lever on the island in the pool of elixirs. Then you go to the bottom left corner of the room and a device is opened which contains the Flutterbird, use the Corrupt Crystal Dampener on it to get it.