Ghost Key

The is an item in.

Location and Uses
After Link defeats the Cubus Sisters in the PH:, they will drop the Ghost Key. When Link enters the ship again and goes down to the first maze with the Reapling, he will find a block with a keyhole in it. After he taps the block to use the Ghost Key, beyond the block, Tetra can be found. The key remains unused for the rest of the game, but Link will still preserve it.