The Ether Medallion is located at a tablet on Death Mountain, just west of the Tower of Hera. Obtaining Ether will require the Book of Mudora and a sword that has been upgraded to at least level 2.
The Ether Medallion uses 32MP (full magic bar is 128MP) to cast a freezing spell over the screen. Some enemies will immediately be killed by it, and others will turn into ice.
This medallion is necessary to access Misery Mire without major glitches.