Heat Blade is the 3rd weapon available to Dart. It can be purchased for 150 gold at the weapon shop in Kazas, and can be found in a chest in Kashua Glacier in disc 3. It deals fire damage, dealing double damage to Water element enemies and bosses, but half damage against fire enemies.
It deals exceptional damage in both boss fights with Lenus in disc 2, and against Windigo in disc 3, and against the Water enemies in the areas preceding them, the Undersea Cavern and Kashua Glacier.
Though you may have access to the newer Falchion for Undersea Cavern and the 2nd boss fight with Lenus, the Heat Blade will deal more damage to Lenus and Regole. A level 21 Dart using a maxed Volcano or Crush Dance will deal 247 damage with Falchion, or 331 with Heat Blade.
The same is true in disc 3: Though you may have the newer Mind Crush for Kashua Glacier and the boss Windigo, a level 27 Dart using a maxed Volcano or Crush Dance will deal 395 damage against Windigo with Mind Crush, or 496 with Heat Blade. As such, this weapon can be found in Kashua Glacier in a chest, as if to remind you about elemental damage.
When using Dart's Special Dragoon command, this elemental weapon benefits from the 50% increased fire damage modifier when using a dragoon addition.