The Fiery Greatsword is a sword crafted from Hellstone Bars. It may have a slow swing speed, but it has the third-longest reach and second-highest damage of all swords before Hardmode. It has a 50*1/2 (50%) chance to inflict the On Fire!debuff for 3 seconds and emits fiery light particles when swung. It is one of the key ingredients in crafting the Night's Edge.
It is the strongest of the swords used in crafting the Night's Edge, in terms of base damage.
Through "usual" game progression, this is the last of the Night's Edge ingredients attained, requiring a trip to the Underworld and some hazardous mining. That said, it is entirely possible to make an early Underworld run and get the Fiery Greatsword even before killing Skeletron and searching for the Muramasa.