As stated above, this is much like teeth of the earthmother. Lets start with the base 3 plus 2 for metal add 2 for voice and you have a spell that creats spike in a helm of about the length of quarter to half inch nails. Because lets face it there isn't enough material in a helm to creat a spike that will puncture the skull without looseing so much material that it would be unefective. Remeber that muto is still effected by the amount of material you have to work with.
Helm of Fangs
Mu/Te 15
Base 3, +2 Affect metal, +2 Voice
This spell creats small smikes inside the targets helm punctuing his skin and causeing +5 damage not soakable by armor, also the abrasions cause the target bleed into his eyes reducing all sight nessicery rolls by -2.
Now lets make a spike capible of killing, take the same base and adjustments, add 1 for creo requisist to make up for lost matieial. Add 4 for increased damage or for unusual/ fancy effect take your pick. And you have a spike that deals +30 damage to the target ignoring armor. The resoning behind this is I have yet to have found a spell that deals out +30 damage that is not level 25 plus. Mind you there are lower level spells that can kill but they tend to require more creativity of the caster or multipul spells. For instance makeing a pit under someone then filling it with sand.
This spell morphs a 6 inch long spike out of the material of the tarkes helm. Creo Requisit is nessicery to add the extra necisery material. It causes the target +30 damage without armor soak usualy killing the victim. If the victim does live roll a stress stamina roll of target 9 if the roll fails the target suffers brain damage and inteligence drops by -2 on a botch the targets inteligence drops by -5. This could also cause blindeness deffness, or mute the target on storyguide descression.
And there you have my take on this spell. It realy comes to a question off balanceing, look at other leathal spells in the book and that is about the level they should be at in my opinion.
My apologies, you are correct. Muto can effect size, so omit the creo req above and change the level to 35. Reguardless, my point was that when makeing high damage or instant death spells you should try to balance them with other spells of simalar effect.
The question is can the helmet be made naturally to have spikes that are directed inwards?
The Anwser: Yes
Hence it is a Re effect. Now to produce the effect, the Rego effect requires a Muto requesite which brings us to a ReTe(Mu) effect.
Teeth Of the Mother Earth should be a ReTe(Mu) effect also but the writter seem to have wanted to place it under a pure MuTe effect with +2 mag for complexity. I would say that that spell was produced by a Muto specialist that had undergone a positive twilight effect which allowed him to create this spell without Rego. Note how unnatural the pillars are. they are automatically white & the top breaks off to prevent escape. This reinforces the Muto aspect.
With Rego a mage can do, what a mundane artisan can do. (Helmets do not naturally grow spikes.) But he needs the raw materials (and not the tools). Hence it is ReTe and the mage needs a piece of iron (or brass, bronze, etc) and a finesse roll.
Hermetic magic does obey physics! Or at least, its own notion of physics within the Medieval paradigm. For example, we know Newton's laws don't apply to Ars Magica magic, but that's not because magic ignores physics, but because Newton's laws don't exist within the Medieval paradigm. Weight does, however.
Basically, a Hermetic spell at a certain level has precisely the power needed to do what it is supposed to do when not interfered with. When interfered with in some way, it will fail to perform fully.
Cases in point: Object of Increased Size cannot tear itself apart through the expansion. Unseen Arm is not powerful enough to yank something out of someone's grasp (nor to crush someone nor lift elephants standing on the object).
Just because a spell is powerful enough to create spikes inside a helmet doesn't mean that it's automatically powerful enough to penetrate the skull of a person inside it. It might be, but you can't assume that just because it's magic, physical restrictions mean nothing.
In the helmet case, I'd say that the base Rego spell can do damage approximately like the ReHe spell Dance of the Staves which also causes a piece of equipment to "betray" the owner - or a bit more because of the head target. With a Muto requisite, you could get sharpness enough to do damage more like a sharp weapon would. Alternatively, increasing the level might provide force enough to crush the skull - either insta-killing (high level) or a high damage bonus.
As for objects materialising inside others: I think the easiest way to deal with that is to simply state that it never happens, but that any magical occurence - be it Creo, Muto, Rego or Perdo - happens during a finite time, in which interfering objects are displaced (possibly very fast) or torn apart, depending on the power and design of the spell. Air displaces easily, costing nothing in terms of spell power. The same case might be made for water. Earth need be either forcefully pushed aside or destroyed (or included in a transformation). Living matter or other distinct objects would have to either be pushed aside forcefully, or destroyed, or the spell might shift its target location (as with a well-performed teleport spell) - or the spell might fail entirely. No interpenetration takes place. Mind, this is just a convenient stance to adopt, I'm not saying the rules say it works this way.