why is a magical flaming greatsword like hes wielding?
He has just a regular greatsword, but his like any of our weapons can be dipped into fire to do extra damage and look slightly cool, its a bonus action but the boss just starts with his weapon on fire. The Greatsword is better than wielding a longsword two-handed because there are 2 dice rolls involved and supposedly the Fighter's Optional Class Feature, Great Weapon Fighting will allow you to re-roll bad damage rolls such as 1s and 2s.
