There are a handful of items that Link finds only in dungeons. All of these items (except the Heart Container) can only be used in the dungeons in which Link finds them.
Big Key
Most dungeons have a Big Key that Link must find before he can open the door to the boss’s lair and fight the boss. The Big Key is one of the last items Link finds in a dungeon.
Compass
Each dungeon’s Compass shows the location of any unopened treasure chests on the Dungeon Map. It also shows Link’s current position and the location of the dungeon’s boss.
Dungeon Map
The Dungeon Map gives you an in-game floor plan for the dungeon. Rooms you’ve explored are light, and rooms you haven’t visited are dark. Use the buttons on the left to view each floor of the dungeon.
Heart Container
Link receives a Heart Container in most dungeons after defeating the dungeon’s boss. Pick up the Heart Container to completely fill Link’s Life Gauge. The more Heart Containers Link has, the more damage he can absorb before fainting.
Small Key
Small Keys are used to open locked doors. Each dungeon has exactly as many Small Keys as it has locked doors, and Small Keys can only be used in the same dungeon that they are found in.