The Wind Fish's Egg is the final dungeon in The Legend of Zelda: Link's Awakening. The egg houses a whale-like creature known as the Wind Fish and is found on top of Mt. Tamaranch on Koholint Island. It can be seen on the title screen, as well.
Link can enter the Wind Fish's Egg only after he has acquired all eight Instruments of the Sirens and plays the "Ballad of the Wind Fish". The inside of the egg is a maze, much like the recurring Lost Woods. Each room has an entrance and three choices for exits. The directions for the correct path can be found in the book Dark Secrets And Mysteries Of Koholint in Mabe Village Library. The book, however, can be read only if Link has completed the quest for the Magnifying Lens.
As part of the Link's Awakening DLC, the Wind Fish's Egg appears on the Koholint Island map in Adventure Mode. The main goal of the map is to collect the eight Instruments of the Sirens Item Cards to dispel the Nightmare plaguing the Wind Fish.