Medallions
Three medallion spells exist. Two are needed to enter Dark World dungeons; one is optional.
Bombos Medallion
Read the tablet in the far southwest of the Light World, below the Desert of Mystery, using the Book of Mudora while holding the Master Sword. Bombos scorches every enemy on screen and instantly melts Kholdstare's ice. It is optional and opens no dungeon.
Ether Medallion
Read the tablet on the plateau atop Death Mountain, west of the Tower of Hera, with the Book of Mudora and the Master Sword in hand. Ether is required to open Misery Mire: stand on the pad outside and cast it.
Quake Medallion
Head to the Lake of Ill Omen, a ring of stones in the northeast Dark World. Throw something into the water to summon the Catfish, who gives you the medallion (no book or sword needed). Quake is required to open Turtle Rock.