Boss Key

A Boss Key is a large, single-use key found in most dungeons, usually in a Treasure Chest. As their name suggests, Boss Keys are used to unlock the boss door in a certain dungeon. They appear in nearly every game of The Legend of Zelda series and first appear in The Legend of Zelda: Link's Awakening, where they are known as Nightmare Keys. Boss Keys are very similar to Big Keys, which can also unlock a large Treasure Chest unlike Boss Keys. Boss Keys usually have a skull motif.

In The Legend of Zelda: The Wind Waker, Boss Keys are known as Big Keys in the English version and as such are functionally identical to the ones in the Nintendo 64 games in that they are found in a large, ornate chest and open a doorway straight to the boss. They use a purple, horned eye design that is used for Boss Keys in The Legend of Zelda: Phantom Hourglass and The Legend of Zelda: Spirit Tracks.