The Firestorm is a regularly occuring natural disaster that plagues the skies. No one knows where it came from or why it exists, but they do know that at some point in the past, many centuries gone, something magical went terribly wrong. Historical records and legends make no mention of it until The Age of Magic, where it suddenly appears, destroying everything in its path.
The Firestorm seems to be a normal thunderstorm at first, though it does not always follow the typical rules in regard to wind or elevation. This storm hunts. Some who study the phenomenon speculate that it has an intellect of a sort, though perhaps an animalistic one, as it tends to seek out places where it can do the most damage.
When aroused (though none know what triggers the change), the Firestorm erupts into a hellish conflagration of dark red flame and searing, tornadic winds. At its center, this gale of combustion can turn even the earth to glass. Anything at the edge fares little better, for many cities large and small have been erased thanks to this infernal, ravenous tempest.
Sometimes, it remains small, dwindling as a spring squall might in a matter of minutes. Sometimes, it grows beyond comprehension, swirling and burning for days at a time before suddenly vanishing. The Firestorm is nothing if not unpredictable, leading many to question if certain Gods were not responsible somehow for its genesis.
Fortunately for the general populace, there is only one of these magical aberrations known to exist, and its appearance is rare. Were it not, the world would be crossed with scortch marks from Fisfen to Luthe. As it stands, most mortals can live their lives safe in the knowledge that they will never see The Firestorm. But even so, the wise look aloft now and again for blood-tinged clouds. And they pray.