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, which calls them Nightmare Keys. Boss Keys are very similar to Big Keys, which are capable of unlocking a large Treasure Chest, unlike Boss Keys.