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 a weapon of choice against monsters based on the water element and it will multiply the damage thus dealing twice as much damage to water enemies.
It also serves as a decent normal weapon in normal fights. 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 Sea Dragon, as this weapon is not to be laughed at.