Mystic Key
From Final Fantasy Wiki
The Mystic Key is a key item in the original Final Fantasy. Many doors in the early portions of the game require its use. It is given to the Warriors of Light by the Elven Prince of Elfheim after he is awakened from the spell of Astos. They are then able to use it to recover explosives from Castle Cornelia to give to the Dwarves so that they may gain access to Melmond. Other rooms opened by this key generally contain treasure chests with powerful weapons or armor for that point in the game, or large amounts of gil.
