The Greater Demon, also known as the Gt.Daemon, is an enemy from Final Fantasy III. It can be found in the Ancients' Maze. It is very vulnerable to Light, thus casting Holy is the best way to go. They can summon Iron Claws, though in the NES version, they summoned Unei's Clones.
Since the Greater Demon will continue to summon Iron Claws until it is defeated, the player can power-level their party by only killing off the Iron Claws as they are summoned, since the party receives Experience points for every Iron Claws that is killed. This is especially helpful at this point in the game, when leveling characters requires tens of thousands of Experience points.
demon is "an evil spirit or devil, especially one thought to possess a person or act as a tormentor in hell."A