Heat Blade is a weapon that only Dart can wield, which is first found at Kazas. It is a very effective weapon not only because of the strength boost it provides over the Broad Sword and Bastard Sword, but because of the fire element that accompanies every slash. This makes the Heat Blade deal 50% more damage to water enemies: 27 instead of 18. It is more or less replaced by the Falchion from Fueno (26), and completely replaced by the Fairy Sword from Ulara (39).
It is not very useful in various locations such as the Undersea Cavern and Kashua Glacier where it can be found; the Falchion is only one point less, but does that damage to non-Water enemies, which are in significant numbers in both locations, and the vast majority elsewhere.
Until Fueno, however, it serves as a decent normal weapon, even against Fire enemies. The Heat Blade's damaging system with water element enemies works with bosses as well. Lenus will be cooked in a few minutes if this weapon is used in both fights. This also works with her Dragon Regole, as this weapon is not to be laughed at.