It would depend on a few things. How effective is the ability? How common
are the mages? What kind of precautions could negate this ability? What else can you do with this?
We’ve talked about how you build armor around the threats you’re most likely
to encounter while using it. If this is an extremely common ability, and one
that can affect entire groups of enemies at once, then, yes, it would seriously
affect the role of metal in combat. Though, it might not mean abandoning metals
So, let’s pick apart those questions and talk about what the mean for your
setting, and your question.
The biggest question is about how well the abilities work. Both the speed of
the ability, and its scale will directly affect how the ability needs to be dealt
with, if it does at all. If it’s on a large scale, torching an entire army at
once, for example, then the casting time (or the speed that the spell heats
metal) only matters if it’s long enough to find and kill the caster.
If the scale is small, one or two people, then the biggest threat would
(probably) be during combat. In that context, we’re back to kill the mage. This
is especially true if the mage needs to be in direct contact to make it work.
Even if they can simply zap whomever they see, they’d be limited to an area
denial role. That is to say, they could prevent hostile forces from rushing
corridors or streets that they’re watching. This also assumes there’s little to
no strain on the mage. If casting this is a strenuous action, and they’re
limited to a couple of zaps, it’s entirely possible they wouldn’t affect
warfare much at all.
If fire mages are exceedingly rare, either because it takes years of
dedicated training, because most people simply don’t have the ability, or
because mages suffer serious attrition during training, that means even large
scale burns won’t affect much.
Think about it this way, if there are five people on your world who can
instantly charbroil an enemy army in their own gear, that’s simply a threat to
be carefully tracked, and neutralized, before you start a battle.
As you add more (and the abilities become more common) it becomes harder to
keep track of enemy mages until you get to the point where it’s functionally
impossible to track them individually. Depending on your setting, that number
could actually get pretty high before you reach that point.
Also, with larger numbers, the smaller scale versions of the ability would
have more of a chance to affect how warfare works. If you’re able to field one
or two mages in your average army, and their primary role is as snipers, that’s
not going to affect how people fight, for the most part. (Though, it could,
seriously, alter how nobles behaved on the battlefield, or even if they’re
present at all.) But, if you can field entire squads of pyromantic infantry,
then those small, “reach out and torch someone,” abilities become a lot more
threatening. At that point, eliminating them before the fight is basically
impossible, so your setting would need ways to deal with them in the moment.
The hard part about introducing magic to a setting is establishing its
rules. To an extent, you need to build an entire set of metaphyics for why
magic works the way it does, before you start getting into specific abilities.
In the absence of that, you have a setting where people will (or, should) work
to counter the threats they face, and magic becomes the convenient answer for
all of life’s problems.
With fantasy, this isn’t automatically a problem, but it is something you
need to keep in mind, when you’re building your world. Look for systems to limit
how your magic works, and what it can do. Or, be ready for a setting that is
very difficult to work with, because the answer can always be, “magic.”
People are creative. When faced with the prospect of being cooked to death
by an enemy mage, the immediate solution is to find a way to prevent that, or preemptively
return the favor. This could be as simple as booby-trapping your soldiers (or
their gear) with spells that will redirect pryomantic magic back at the caster,
or enchanting their gear with some kind of thermal negation effect, so when the
pyromancers try to flash fry them, all they manage to do is give their foes
flaming weapons and armor.
This could also lead to armies making extensive use of divination, allowing them
to better track enemy mages. Which in turn would lead to mages looking for ways
to create decoys, moving around forces that don’t exist, in an effort to
confuse anyone scrying for them.
It could also result in the creation and enhancement of other materials that
are magically immune to pyromancy, or made from something the pyromancers can’t
affect. Such as impossibly durable resins, or unmelting, super-hard ice plates.
It’s also worth realizing that these kinds of powers would radically change
the way metallurgy developed as a technology. When you have mages that can replicate
forge technology that wouldn’t “naturally” exist for centuries. There’s a lot
of potential for changing the way it evolves. You could very easily see much
higher quality steels than the real world ever produced in its analogous era,
and potentially even alloys that simply aren’t possible in the real world. I’m
not sure what 12th century battlefields would have looked like with space age
alloys, but it’s not outside the range of possibility for your setting.
Especially if your pyromancers can participate in the refinement process as
well. This also leads to the potential that they may have materials that are
centuries ahead of their technology, (because magic allows them to work with
the mater directly).
When you’re creating a world, and you come up with an idea, usually, the
best thing to do after that is sit there, and see if you can find all the ways
people would react to it. An ability like being able to instantly heat metal to
forging temperatures would be dangerous in combat, but it would also have many other potential applications.