Celerism

Celerism is the next step beyond Pyromancy, and the most advanced verison of elemental fire magic. Generally used by

Nova Magic
Almost entirely unused, and not for lack of destructive potential. Nova magic requires Celerism and Velocism at very high levels, as the principle is to work with space and motion to split and smash atoms. In short, this is a magic based entirely on nuclear explosions. There is literally no way to use this in a way that doesn't kill everyone around the user, and probably the user too. The amount of time it takes to train up to this level, the fact that there is no safe way to train in it, and the whole "radiation poisoning in the giant crater you made of a town several miles that way using only the barest sliver of magic" thing, coupled with the tremendous HP cost of any of the spells, means there are no Nova mages.
 * Atomic Bomb: Yeah, that kind. No, there's nothing smaller. Good luck being able to cast it far enough away not to kill you.
 * Hydrogen Bomb: It gets bigger. Hope you like nuclear apocalypses!
 * Supernova: So you've somehow managed to defy all logic, sense, and survival instinct and assembled a bunch of Nova mages despite everyone trying to kill you. Okay. Give me the lives of 20 Nova mages and you can annihilate your solar system. It's not an actual supernova, as training the entire population of Itaedia in Nova magic wouldn't be enough, but will you know the difference? Nope! You'll be dead!