[No, you declare your attempt to cast a spell on what you wanted to cast it on. if it's a spell I've never seen before, then describe the desired effect so I may know how to effectively fuck it up when you rolled a critical failure. The more complicated and reality-bending spell that is, the more ways it can fuck up. A thing as simple as creating a fireball by magically-induced atomic fusion may only fizzle or blow up in your hand, but it won't, say, summons a daemon from the immaterium to try and corrupt you with psychic shotgun blasts.
It also depends on the scale, of course. An explosion of a baseball-sized fireball isn't going to be anywhere near as big as ones the size of a car that you've been cooking up for two turns. There are ways to avoid catastrophic failure of course, such as making precaution wards for your rituals or wearing a sort of magical capacitors to help soak the potential fuckery. Having at the very least a minimal of those could very well prevent an accidental nuclear explosion from a bad atomic-fusion spell gone bad. Or accidentally ripping a hole through realspace and invite a whole bunch of void beings from the outside to come and fuck absolutely everything up in the most horrible ways.
So yeah, no pressure!
Note: Spells and Rituals usually cost something, Rituals cost reagents, with the more complicated and powerful rituals using more reagents, this is the 'safer' way of casting spell where even beginners can do with less chance of critically fucking things up if they follow instruction correctly.
Casting an Arcane spell is a more 'traditional' way of casting, more like your usual fantasy sorcerers and wizards. This too could require reagents or a sort of power that is accepted, Akras finds life-force or souls to be an easily obtainable ones with their literal endless supplies of clones to draw from or sacrificial monkeys. Blood is also often an acceptable medium for the cost, yours or not. It's harder for your characters to keep track of how much 'magic' you have left to cast, unlike modern firearms with helpful numeric display of how many bullets are left in the magazine, so I will only be telling you when you feel if the powers are running low or if the spell is nibbling away at your very being to force reality to it's will.
Basically, when in doubt, make a bigfuck magical circle, design it to do what you want happens then sacrifice a hundred virgin clones to it and hope for the best. Akras have subdued a greater entity using literal field of sacrifical monkeys after all. Just... brute forced it to submission with monkeys upon monkeys upon screaming monkeys.
Said greater Entity have Monkeys PTSD now.
You can ask Kevak for more info about Magic and stuff, he's helped me expanded the system.
Also, Relevant. For that very special critical failure.
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