Casting is not one decision. Every spell asks you two separate questions — how hard are you throwing yourself at this, and how much are you putting into it — and they are paid for out of two different pools, for two different returns.
A cast spends both, always.
The stamina half works exactly as it does for someone swinging a sword: you choose how hard to push, and the extra dice go on the casting roll. It buys you accuracy — whether the thing lands, and by how much.
The mana half is the one that has no equivalent in swordplay. Every spell has a cost, and you may choose to pay a multiple of it: twice over, three times, as far as you can afford and as far as you are capable of. That does not make the spell easier to land. It makes it bigger.
It depends on what kind of spell you are casting, and the three answers are quite different.
Something that hurts one target simply hurts it more — the spell's own damage is multiplied by what you paid. Note what is not multiplied: the bonus you earn for rolling well. That part stays as it is, deliberately, so paying more gives you a higher reliable floor rather than compounding with a lucky roll into something wild.
Something that reaches a group reaches more of them. The extra mana buys additional targets in the rank you were already aiming at — never spilling into a different group, and never hunting for someone else if that group is smaller than you paid for. You roll once, and every one of them defends against that same roll separately.
Something that mends mends more.
And some spells cannot be pushed at all. Whatever they do, they do once, at their price.
Your training in that spell's domain is how far you can push a spell and be sure of it. Two dots, twice over; five dots, five times. Within that range the spell simply does what you asked.
You may ask for more. Nothing stops you, and the game will not tell you that you have crossed the line — you find out when the power arrives, or does not.
What answers when you reach is the part of your Aether your training has not yet caught up with. Aether is raw attunement; training is what you have learned to do with it. The gap between them is the reservoir you draw on when you go past what you know, and the further past you go, the harder it pulls back. You keep what you could hold — reaching for five and holding three is an ordinary outcome, not a failure.
Two consequences worth seeing. A caster with deep Aether and thin training can reach a long way, unreliably — there is a lot of untapped gift there. A caster whose training has caught up with their Aether has nothing held in reserve, and reaches on the barest chance.
And at the top of a domain the question disappears: a fully trained caster is sanctioned across the whole dial, so there is no test left to fail. Training does not beat this — it retires it.
Spreading yourself across the three domains still costs more than it looks. Dots in three domains give you three short reaches. Dots in one give you a caster who can put their back into something and know it will land.
These are separate, and the distinction is the whole shape of a caster.
Knowing a spell is what lets you cast it. You learn spells — a few when the character is made, more from anyone willing to teach — and what you know is what you can do. Training in the domain does not hand you spells.
Training in the domain is what makes it work. It is half your casting pool, it is the whole of how far you can push a spell and be sure of it, and every dot raises your mana besides. An untrained caster holding a known spell can throw it and will mostly fail.
Every Cast menu lists everything the caster knows, every time. It is never trimmed to what looks sensible right now, and that is deliberate — the game does not decide on your behalf what is worth trying.
You do not have to be in a fight. Spells can be cast while exploring, and doing so takes a moment of the party's time like anything else.
But read the previous paragraph again, because it has teeth here. The menu will happily let you cast a battle spell at an empty corridor, take your mana and your stamina for it, and give you nothing. The cast genuinely happened; there was simply nothing there for it to happen to.
Only some spells have anything to offer outside a fight. Knowing which of yours are worth casting in a quiet passage — and which are being kept for when something is standing in front of you — is knowledge the game expects you to acquire, and it will not warn you first.
Not the way stamina does. Stamina returns as you walk and between rounds; mana does neither. It does not trickle back in a fight, and a long walk home restores no mana.
What refills it is resting properly: a camp, or a bed in town. That is the whole list.
So mana is not a per-fight resource that resets. It is an expedition-long budget, and every cast spends a piece of it that nothing will replace until the party stops. A caster who empties themselves in the second room of the night is a caster the party is escorting for the rest of it.