Ars Mundi: Coldfire

The Great Archmage Talistandra Orvalus XII is credited with the discovery – posthumously, of course. Her death baffled the College for months before enough of the event was reconstructed using divination and scrying magic. Orvalus' remains were found in her tower, along with the burnt remnants of her books, clothes, bookshelves, wands, staves, cat, owl and collection of novelty tea cosies, all scorched and clearly destroyed by flames. Yet when her apprentice arrived later that day, he found the entire tower covered in a thick layer of frost, even though it was a pleasantly balmy day in early spring.

Like any true academic, Orvalus was absolutely delighted about her discovery, even as it burnt her alive.

What the Archmage had discovered was the polar opposite of fire – coldfire.

Coldfire acts very much like fire. It's a chemical reaction that converts chemical energy in fuel into heat via oxidation, the heat then going on to provide the activation energy for yet more reactions, repeating ad infinitum until there's either no fuel left or no oxygen left. Except coldfire converts chemical energy into cold. The reaction is self-sustaining, the incandescent, freezing gases teasing more energy out of fuel to create more and more cold. A wooden log will catch coldfire and burn completely, eventually turning to ash as the wood consumes it. But the ashes will be freezing cold! Water vapour in the air condenses around the flames and crystallises into ice granules nearby, forming "smoke". This created cold is nonmagical – if the ambient temperature is warm enough, a forest burnt down by coldfire will look just like one burnt down by normal fire then rained on.

The discovery of coldfire threw the elementalist theories of the universe into disarray, as they had always maintained that water was the polar opposite of fire. Some enterprising elementalists have tried to preserve the theory by proclaiming that each element has a corresponding dark element and that coldfire is simply dark fire... Critics point out how ridiculous it is to call something "dark" just because you don't know anything about it, as well as bringing attention to the complete and utter lack of evidence for the other dark elements.

Coldfire Spells
The coldfire college is exactly like the fire college (GURPS Magic, p. 72), except that damage is cold, not heat. However, don't remove the incendiary damage modifier from the attacks – coldfire can still ignite flammable objects! Anything ignited by coldfire continues to burn with yet more coldfire, spreading as natural, normal fire would.

Using Coldfire
Coldfire might be the signature magic of a particular group, perhaps a villain. If the heroes are never there when this group assassinates nobles or burns down farms, then it adds to the mystery – how is everything burnt and frozen? Coldfire's origin might be extradimensional: perhaps mortals of the Prime Material Plane can't grasp the alien physics and chemistry necessary to work coldfire magic, so only outsiders can use it. In this case, it's good as a marker in dungeons and quests. Just like bite marks mean werewolves, traps mean kobolds, and bleeding necks mean vampires, coldfire means extradimensional wizards!