Flamethrower Blade is an object unobtainable by players through normal means. This item's information is present in the game files, but not intended by developers to be in players' possession.
Flamethrower Blade is a unique flamethrower which is used by the challenger in the third and final arena challenge at the outpost. It's simply a flamethrower that looks like a sword which he toggles to during the fight.
This flamethrower isn't available to players through normal gameplay, but defeating the encounter will reward Firestorm's Fury which has an identical model and a flamethrower secondary attack.