Bombos Medallion
"Bombos
The ancient Hylians built giant monoliths which were meant to instruct the Hero about very special magic spells. The Bombos monolith was placed on a cliff above the entrance to the Great Desert. By activating the Bombos Medallion, Link can create a large explosion around himself which will defeat a group of enemies.
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The Bombos Medallion is the only medallion not required to complete the game, but is arguably the most useful one. While consuming a large amount of magic, the Bombos Medallion can defeat all the enemies on the current screen.
To acquire the Bombos Medallion, head towards the swamp in Dark World and use the Magic Mirror to appear on a cliff in the Desert in Light World. Go left one screen and use the Book of Mudora to read the tablet.