A fiery greatsword is 80% better than a typical longsword, despite its slower speed. It is much heavier than a longsword. It is 30% better against larger foes than against others. You will require practice with greatswords to use it well. It will burn foes a quarter of the time you hit them.
You cannot dual wield Greatswords but a shield can be equipped with them, which is highly recommended upon initially using this weapon type as it is immensely slow in terms of attack speed compared to anything else (providing you have the strength, a vandal/barbarian start may be able to). It's a great addition to a Knight build with the additional fire damage over time. It can be found randomly.
Using it effectively requires a skill of at least 2 - lower than this it will be wildly inaccurate and much worse than a normal short sword.