Daedric Greatsword of the Inferno is used by conjured Dremora Lords. Dremora Lords do not drop them upon death but they can be created by the Dragonborn by smithing or acquiring an ordinary Daedric Greatsword then enchanting it with an appropriately leveled "Fire Damage" enchantment at an arcane enchanter. It is also possible, although unlikely, that one could spawn as random loot, which only happens at a high character level.