Boss Key

The Boss Key is a necessary dungeon item. It actually goes by several names: Boss Key, Big Key, and, in Link's Awakening, the Nightmare's Key. It is used to open the boss's lair and other big doors. In A Link to the Past, the Key is also necessary to obtain the item hidden within each dungeon. In Phantom Hourglass the Big key must be carried by Link to a huge golden key block with an eye embeded on it.

This Key has been used in almost every Zelda game to date, and most likely will be in future games as well.