So I've pretty much done the analysing and theorising (slash pontificating) side of things, and I really need to just actually, you know, design a magic system.
I wasn't even going to get into powering spells until after I'd done most of the other stuff, but an exciting idea turned up in a chat with Dan, who calmly said something like "why don't you just tie spellcasting into body temperature?" as if it wasn't some kind of stroke of genius. I think it may be, actually. If it works out, this could well handle a whole slew of things all at once, as a side effect of powering spellcasting.
So, spells drain body heat from you to provide their power; or at least, as a consequence of casting you get colder. One obvious consequence of this is that it provides a casting limit. You can only cast spells while you're warm, and either caution or sheer physics will limit what you can do, due to unwanted falling unconscious.
One really nice thing is that it gives a big boost in importance to the temperature management system, which is great because I'm keen for that to be a meaningful part of the game, especially since it's a unique feature. It also makes it more interesting, because now rather than just temperature, there's an interplay of factors to think about.
Of course, if you're in a room above freezing, then technically you're going to be able to cast unlimited spells given enough time. This is only really an issue if heat transfer is rapid and spellcasting is rapid. Otherwise, we can just trust that it won't really be that beneficial compared with other things you could do in that time. If cooling down is a serious problem, and warming up takes a while, then trying to exploit this becomes worthless. Not that I particularly expect people to be breaking Monitors, but I don't want to design in flaws unnecessarily.
On the other hand, if you're in a hot room, then casting spells will actually enable you to cool down. This is potentially quite a cool (no pun intended) mechanism, and I quite like the idea of overheated lizards grimly chanting away in a desperate attempt to stay conscious by channeling unstable arcane energies.
There's obviously details that need fiddling with. How many therms can you spend on a spell, or within a single round? Having characters drop their body temperature by thirty degrees in a few seconds seems a bit extreme, but you could allow use of powerful spells by spending therms over a number of rounds. This would provide a way to limit more powerful spells, by imposing a logical time-cost, as well as potentially allowing for very powerful rituals where heat sources are required over long periods. I'm inclined to say one or two therms per round would be as much as you could reasonably spend, unless I wanted to introduce rules for risky overspending.
An important note: if I use this system, I need to prevent using magic to restore body heat, otherwise we're getting into limitless magic territory by accident, while pretending not to be there. That's not going to work out well.
Mammals and spellcasting
Another vital issue is the effect of endothermic spellcasting on non-poikilotherms. One simple option would be to translate therms directly into damage, which is likely how I'd deal with environmental heat and cold. This would handily boost the setting's herpetocentricism by providing another mechanical reason why reptiles are Better At Stuff: they can handle magic much better than mammals because they can withstand its physiological effects more readily, and this once again means they're a good option for special response teams.
Mammals would still be able to cast spells, but in most cases their capacity is much more limited by the pain incurred through spellcasting. Some non-reptiles would also have magical capabilities (fish, some insects, some aliens), and might have variable heat capacities in the same way as the reptiles.
So basically, this seems like it would usefully:
- Provide a mechanic for powering spells and limiting spellcasting
- Provide a mechanical cost to spellcasting
- Provide a fluff sense of cost to spellcasting
- Reinforce the narrative favouring of reptiles in the setting
- Increase mechanical differentiation between poiks and non-poiks (and degrees of poiks)
- Increase the mechanical importance of temperature regulation
- Improve integration of temperature regulation with other mechanics
What's not to like?