Since I seem to be using de facto categories of spell, I should probably try to thrash out what those are. This would allow me to look out for potential issues, and also to consolidate some rules by applying them to categories rather than individual spells. I will attempt not to just reel out the names of spell schools in D&D, but damme if they didn't do a pretty good job nailing that stuff.
A secondary but important goal is ensuring that category names are nice and distinct, rather than, for example, all beginning with the same letter. Right, FATE?
Conjuration spells seem to be fairly prominent, and because there's historically some issues with those across games I should pay some attention.
I'm also fond of spells that involve runes and other big glowing things you can carve into the very air! that do stuff, so let's consider those a category of Sigils.
While it's very fluffy, a lot of spells are going to fall into a generic category of pretty generic spells that mostly have immediate effects and neither summon things nor create runes. Let's call these Invocations.
I haven't yet offered any examples, but I imagine some kind of Transformation spells are likely to come up. To avoid the Polymorph Problem I will want to pay these careful attention.
Conjuration
Conjuration spells allow you to summon an entity that acts semi-independently, typically having its own dicepools and instinctive behaviour.
Conjuration spells vary in power, requiring different amounts of energy (heat points) to cast them. These points are not merely spent, but invested. The investment is determined by the power of the entity summoned, which ranges from Trivial to Minor to Major. A single casting may summon several Trivial entities, while summoning a Major entity will drain multiple heat points over multiple rounds.
I had a lot of concerns here about limiting the power of summoning, since a wizard in a warm room could in theory summon an unending army. I spent ages working out possible ways to limit this with heat so that Conjuration spells don't create a massive advantage in either hot or cold conditions. Some failed models are listed at the bottom of the post.
Sigil
Sigil spells inscribe a symbol or create a field that provides an enduring effect. In essence, once the sigil is written, it continues to cast itself without the wizard's intervention.
Sigils take varying amounts of time to inscribe and may have varying power requirements. A wizard can always disable their own sigil without requiring a roll, though they must be within a specified range to do so. Some sigils are indescriminate, others can affect only specific targets. Because a sigil is dependent on shape, they can be rendered ineffective by altering the sigil or manipulating the surface it is inscribed on. Some sigils can be muffled by covering them with cloth or paint, but this is not a universal solution.
All sigils require the investment of the heat points expended to cast them.
Invocations
Invocations are the most generic type of spell and the basis of the magic rules. They have no additional special rules.
Transformations
Some spells can alter the physical form of a target entity or object.
Only the most recent transformation remains active, so a creature cannot benefit (or suffer) from multiple simultaneous transformations.
A wizard attempting to replace an existing transformation must make an opposed Wits roll against the original caster.
Some Transformations require investment of heat, but not all.
Investment
Any heat points currently invested in spells are recorded alongside the body temperature chart. If the wizard's body temperature plus their invested heat reaches 11+, they immediately fall into heat shock. Thus, it is dangerous to try and maintain large numbers of spells by remaining near a heat source.
Broken models
For reference, here are some abandoned models of how to handle investment in Conjurations (which I later extended to Sigils for similar reasons). The original point of this mechanic is partly to be atmospheric, but mostly because otherwise a wizard with a heat source can conjure an unlimited number of entities, losing heat each time and then regaining it. I felt it would be preferable to a hard cap. Eventually I went for the option described above.
Distance model
For each point currently invested in a Conjuration, the character's body temperature is treated as being one step further from the ambient temperature when determining temperature change.
Problem: in a warm environment this exacerbates the problem, allowing the wizard to regain body heat faster than normal. In a cold environment it makes conjurations far more dangerous than other spells for no obvious reason, since the character will lose heat rapidly and potentially become torpid even in a non-freezing room.
Hotter Model
For each point currently invested in a Conjuration, the character's body temperature is treated as being one step higher when determining temperature change.
Example of Investment
Starting from a body temperature of 6, Xerxes uses Fragments of Dreams Abandoned to conjure a Splintered One. He invests two points in the entity over two rounds. His temperature is now 4 with two points invested. His actions are subject to the rules for being Chilly in accordance with his temperature of 4.
If he is in an office at temperature 6, then there will be no temperature change. He is treated as temperature (4+2 = 6) which is the same as the office. He will be unable to warm up while he remains here.
If he is on a windswept marsh at temperature 2, then he will roll 4 dice to test for heat loss. He will continue to roll for heat loss until he reaches body temperature 0, at which point he will go torpid.
If he is in a boiler room at temperature 9, he will be sheltered from the heat to some extent as he rolls only 3 dice for heat gain rather than 5. When he reaches body temperature 7, he will stop rolling and can continue to act at optimum ability.
Problem: This model doesn't quite allow for unlimited summoning. When Xerxes has invested 9 points, he'll stop gaining new heat points but also fall torpid because his effective body temperature is 0. However, it allows conjuration spells (but no others) to provide a safe buffer against heat, rather than an immediate cooldown with the risk of side-effects. As long as you have at least one summoned entity, it'll be impossible to go into heatshock. Meanwhile, cold environments become extra-dangerous even though they weren't part of the initial problem.
Instability Model
For each point currently invested in a Conjuration, the character rolls an additional die when testing for temperature change. If body temperature currently equals the ambient, determine whether heat is gained or lost at random.
The more points are invested in conjurations, the more body temperature will swing and the greater the risk.
Example of Investment
Starting from a body temperature of 6, Xerxes uses Fragments of Dreams Abandoned to conjure a Splintered One. He invests two points in the entity over two rounds. His temperature is now 4 with two points invested. His actions are subject to the rules for being Chilly in accordance with his temperature of 4.
If he is in an office at temperature 6, he rolls two dice plus an additional 2 for the points currently invested. When he reaches temperature 6, he continues to roll the 2 dice for his invested heat, and may swing from temperature 4 to temperature 8 depending on the rolls.
If he is on a windswept marsh at temperature 2, then he will roll 4 dice to test for heat loss. When he reaches temperature 2, he continues to roll 2 dice and may swing from temperature 0 to temperature 4, passing in and out of consciousness.
If he is in a boiler room at temperature 9, he will roll 7 dice in total, which means a bad roll could land him in heat shock immediately, though it's unlikely. He will gravitate towards temperature 9 and then swing from 7-11, again passing in and out of consciousness.
Problem: This isn't a terrible model, and generally does what I'm aiming for, but it feels unsatisfactory at the point where body temperature should stabilize. Because of the role of chance (and insulation) it's also relatively unlikely that wizards will actually pass out.
Blocking Model
When a point is invested in a conjuration, that step of the heat scale becomes unavailable and is blocked off, chipping away at their ability to cope with temperature change. The more steps are blocked off, the more vulnerable the monitor becomes to torpor or heat shock.
Example of Investment
Starting from a body temperature of 6, Xerxes uses Fragments of Dreams Abandoned to conjure a Splintered One. He invests two points in the entity over two rounds. His temperature is now 4, and steps 5 and 6 on the scale are blocked off. His actions are subject to the rules for being Chilly in accordance with his temperature of 4.
If he is in an office at temperature 6, he rolls two dice as normal for heat change. If he gains one heat point, he will skip over the missing steps and reach temperature 7. As this is warmer than the office, next round he must roll again for temperature loss. While the points remain invested, his body temperature cannot stabilise in this temperature 6 room.
If he is on a windswept marsh at temperature 2, he will roll 2 dice to test for heat loss as normal.
If he is in a boiler room at temperature 9, he will roll 5 dice as normal. However, any heat gain will skip right over the missing steps, which means a bad roll could land him in heat shock immediately.
This model seemed very promising when I was mulling it over. It has essentially no effect on cold environments, since the monitor's body temperature will drop as with any other spell, and as the tendency will be for them to get colder, the fact that some steps are missing higher up is little problem.
Of course, someone could plan for a cold environment by using up steps 2-4 on the chart through investment, then hoping to find heat sources that will let them leap from 1 to 5 with very little effort, minimising the time they spend on heating up. Bear in mind, though, that they'll be rolling a lot of dice to see if they cool down and even one die can send them back to temperature 1 - so it's not exactly flawless.
In warm environments, blocking some steps will make you warm up faster. You could use this to make it easy to stay at temperature 7, but if you keep summoning more things then you'll lose that slot as well.
Essentially, the problem I was concerned about was wizards in a warm room summoning unlimited numbers of things over a long period. In this model, a wizard in a temperature 7 room will lose the 7 slot. They'll then tend to gravitate to temperature 8; investing that, they'll gravitate to 9, and so on. In a temperature 7 room, a wizard can invest only 4 points of heat before they are at serious risk of passing out from heat shock. Strictly speaking, you could argue that they could wait to wake up and then summon more stuff until the whole 2-10 range is blocked off, but it's very unlikely anyone will try it, not least because they'd be constantly passing in and out of consciousness.
Problem: This model is fundamentally kind of faffy. You have to use an actual chart to block things off, not just a pile of tokens or a die as you might otherwise. It also gets complicated because it convolutes the (fairly simple) heat rules, and raises questions about what happens when a conjuration ends - do you just pick one slot to unblock, or do you need to remember them? While I like it, I feel like it would unnecessarily complicate the game to achieve a fairly simple end.