The Emerald Weapon is one of several Weapon monsters that wreak havoc on the world of Gaia in Final Fantasy VII. The Emerald Weapon is located underwater, near the shore of Junon. It swims around, waiting for an enemy to approach and fight it.
The Emerald Weapon fight is unique in several ways. For one, it has one of the highest amounts of HP of all the Weapons you can battle. He also must be fought underwater, meaning that the party only has 20 minutes before the battle is over and the party dies (from lack of air). That is, unless you have the necessary Underwater materia, which simply removes the 20 minute time limit.
If you defeat the Emerald Weapon, you will receive the item Earth Harp. Trading it to the Kalm traveler gets you the Master Command materia, which allows you the use of all forms of Command materia.