I'm trying to redact the Cruciatus Curse from the Harry Potter books using the spell creation rules, and I think I've got it just about right.
What's throwing me off is the Base Level. I'm thinking that pain is Mentem, as it doesn't really fit any of the other Forms (with the possible exception of Corpus). I don't think it's Corpus, as it doesn't really do anything to the body – no marks, no actual physical effects, the target simply feels pain, which to me seems more Mentem than anything else.
A wand enchanted with the animal equivalent, "Wand of Bestial Agony", is used as an example in (at least) two of the books. Of course, having something possible in Animal doesn't help all that much when deciding whether something should be Corpus or Mentem.
The main reason an effect like this is not more prevalent is because it is Concentration, and it is rare that a beginning magus can afford to Concentrate on a single target spell. It has its uses, but if you are fighting mundanes you are probably fighting a lot of them and you want spells that affect a larger number of targets. It's not a bad single target curse sort of effect, though. And since it is low level it can get good penetration fairly easily. Its best use may be on targets with magic resistance, who you pin with pain while your Grogs chop the thing's head off.
How did base 4 "Cause a person pain, but do no real damage" become "Cause incruciating pain, making any actions impossible"?
What if the wording of base 10 "Cause the loss of a Fatigue Level", instead had been "Make someone weary". Would we have concluded that the target was rendered unconcious ?
I'd require an extra magnitude for increasing the intensity of the damage. That would make it more in line with the base 5 "Hamper a person without actually injuring them".
If not, I would at least call for a Stamina roll of 6+ or 9+ to resist the pain, and if succesful allow the target to act regardless - althought with a -3 to all rolls (double that penalty for Concentration rolls and spellcasting). With this Stamina roll, the target would also benefit from a high Stamina, which i find highly appropriate, as well as a large bonus from Enduring Constitution (which is otherwise very rarely used).
Cruciatus Curse (PeCo20)
Cause excruciating pain on the target as long as you concentrate.
There are also popular versions with Diameter duration (+1 magnitude) and/or that also affect animals (+1 magnitude, An requisite)
B5, +2 voice, +1 Concentration
I would allow that with no problems IMS. A level 25 spell tends to be a total killer against a mundane anyway, regardless of the Form used, so having this spell at level 20-25 to incapacitate a target for a while sounds more than fair.
Because it's not a terribly useful effect, I'd say.
The only two uses I can see are as a minor distraction in combat, or as a means of torture.
In terms of combat, if the spell penetrates, and if the target is suitable (unsuitable targets include non-humanoid targets, humanoid targets of size +2 or larger, targets that cannot feel pain etc.), this atmost* incapacitates the target as long as the magus concentrates. I'm saying atmost* because, if we assume the spell works like Agony of the Beast (which uses a guideline worded identically) a Size+Sta roll of 9+ allows the target to act in a round despite the agonizing pain. Meh.
As far as torture goes, either the magus is a sadist (and I don't think there are so many in the Order) or torture is a means towards an end ... and there are usually far more effective means (in particular Mentem spells).
Don't forget that if you use this spell on someone and let them live afterwards, they'll probably hate you, whereas a ReCo spell based on the guideline 'holds a target motionless' (Base 5), is only one additional magnitude, works better, and probably creates a lot fewer mortal enemies to haunt you afterwards.
I'll politely remind people that the OP said nothing about inventing the world's most effective spell. Nor did it include any min-maxing. Actually the OP was originally inventing the spell with an even higher base, and was inventing it with Mentem although the Magus was actually better with Corpus. This makes me think, that this wasn't all about empowering a certain Magus with the best tool ever...
The proposed spell does something different than all of the above alternatives, that has been suggested. It's less subtle, yes - but that's not neccesarily a bad thing! Do you really think it was the most effective spell Snape could come up with in the world of Harry Potter? Naah - but it is certainly one of the most feared spells, don't you agree? Also, if you are a sadist, or have a focus in pain, torture or causing terror - this is a great spell. I'll put it to good use in my saga, that's for sure. Thanks.
BTW: Many of the spells in ArM could be made into much more effective versions, although they are not. Many spells are widespread within the order, that has not been subject to min-maxing at all. Wonder why...
I think I like the idea of raising the spell to 20 to make the pain paralyzing.
I'll also note that a level 20 spell is right in the ballpark for new magi fresh out of apprenticeship. While the spell is certainly niche, I can see plenty of uses for it in roleplay. If I had a Tytalus magus, he would totally know this spell and hate it ... Because it would have been used on him during apprenticeship!
I agree that this should be PeCo, but that Base 4 is too low.
I'd instead look at what it takes to cause the different Wound Sizes. If the target is rendered totally helpless by pain I'd compare this to an Incapacitating Wound and look at what this takes. Should it be easier or harder to just do pain and no damage? Guidelines suggest it is easier with simple pain. Base 4 compared to Base 5 for a Light Wound. Also Damage is Momentary magic that persists, pain needs Duration. Shame the Core rules don't say what the consequences of this Base 4 pain is. I'd not rate this as more than a -1 to -3 Penalty.
Also consider that the Base 5 covers "Hamper a person, like blurring eyes or making a leg lame" this sort of damage heals like a light wound. Perhaps excrutiating pain needs to heal as well, or are you just good to go once the spell ends? Or cost fatigue?
So Base 20 is for Incapacitating wound, I feel Base 10 seems apporpriate for Cruciatis, +2 Voice +1 Concentration (or more) for a lvl 25 spell. Perhaps adding a magnitude extra for also causing a fatigue level once the spell is over, or perhaps ruling that the pain leaves physical damage which heals as a light wound.
A cool spell if you need to be rally, really mean to someone, or if you need to take an enemy alive. Sure, ReCo is more effective - but supposing you're a follower of Apromor and really suck at Rego?
Cool spells rate high by me, I'm not (always) into optimizing.
This is similar to the one I came up with for an NPC. When I fix my desktop, I'll be able to pull it up. But I also increased the base to 5 to indicate incapacitation and torture instead of just pain. It was a spell and then the NPC enchanted it into a rack with a Touch range...
To me, the base 4 has little mechanical effect, it is more a roleplaying tool, used for torture and such.
It'd probably inflict a -1 penalty, similar to a minor wound (Well, doesn't a minor wound cause pain? And what qualifies as a "minor wound" is quite enough pain for me IRL ), but nothing more, certainly not incapacitating pain.
Now, with added magnitudes...
Well, this is efficient, but blatant. There's the funny one, too.
A ReCo circular ward, plus a ReCo Aq ward.
Feed the target as normal, you can even give him fine food and wines. Don't forget a warm cell, you don't wanna see him catching a cold.
Thing is, where are the excrement gonna go? They can't exit the ring...
Leave him there long enough, he's gonna (litterally) drown in his piss and shit. And he'll see it coming.