Ghost Key

The Ghost Key is a minor plot item from The Legend of Zelda: Phantom Hourglass. It resembles a Boss Key, but purple with a skull-shaped handle with a whispy ghost tail like design on top. It appears once the Cubus Sisters are defeated on the Ghost Ship, with the eldest leaving it behind as she disappears. As such, it is technically the dungeon reward, despite not matching any of the game's other dungeon rewards in terms of theme (unless one counts the Sun Key on the third visit to the Temple of the Ocean King, which does not require fighting a boss) and not appearing on the file select screen. It is used to open a particular large, skull-imaged lock, somewhat resembling a boss door's lock in shape, but with the keyhole on the front instead of the top. The lock is found on the B2 floor of the ship, and once opened, slowly descends while letting off white light. Behind the lock is a staircase leading to the hold where the petrified Tetra is.