Old Mermaid Key
From Zelda Legends Wiki
There are two Mermaid Keys. One opens the door to the Mermaid's Cave in the present, the other opens the door in the past. This particular one is for the past door, and can be won at the Big Bang Game.
There are two Mermaid Keys. One opens the door to the Mermaid's Cave in the present, the other opens the door in the past. This particular one is for the past door, and can be won at the Big Bang Game.