Have the eight Instruments of Siren and the Magnifying Glass, then go to the town Library. Read the bottom-right book and write down the code. Head to Mt. Tamarach, and play the Ballad of the Windfish on the Ocarina. The instruments will play it for a while, then the egg will shake and burst open. The owl will appear and tell you you should go in to decide the island's fate. You enter the egg and follow the directions you had written. You then reach the bosses' chamber.(there is more than one)