Flametongue is an artifact found early in the game that increases the character's Strength by 2. In Final Fantasy Crystal Chronicles: Ring of Fates the Flametongue is a weapon that can only be used by Clavats. It adds 45 to Attack and can be made by using the Fiery Weapon scroll.
Another sword, called the Fire Edge, is more powerful, adding 135 to Attack. It can be made using the Fire God's Weapon scroll.