Ghost Key

The Ghost Key is an item in Phantom Hourglass.

Location and Uses
After Link defeats the Cubus Sisters in the game's fourth dungeon, the Ghost Ship, 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.