If you want a redesign of Parma , Magic Resistance and Spell Penetration ,
you may need to provide some guidelines for what effect you are actually trying to achieve.
I agree that the old type ablative parma and the new penetration clash - been my point all along. But just as I wasn't interested in including the old type ablation, though wanting to play along as an experiment, I'm not interested in rewriting the penetration. I think it works bloody well as is.
I still think that the best answer is the PeVi and breaking down the mechanics presented in Winds of Mundane Silence. It would read something like: This spell erodes a target's Parma Magica by the number of points the spell's level exceed the targets Parma Magica x 5. This spell naturally needs to penetrate.
If you want a generel rule, where all spells wear and tear the parma, yet not wanting to encourage low level spells (to boost penetration), you could rule that if the attacking spell at least equals the defenders Parma Magica Ability Score, then the surplus Penetration is subtracted from the defenders Parma until his Parma ritual is redone.
You could also opt to use the Ablative Parma Ravenscroft is refering to. In terms of RAW this kind of Parma is seen as a Breakthrough know and taught within the House of Bonisagus.
It's not that I want a redesign of all of those systems. In fact, I would prefer not to make a such large changes. I was just trying to say that, if we do chose to redesign one of those systems we have to recognize that it may require redesign of the others. They are so interconnected that it would be silly to think of them as different systems. Rather they are all different parts of the spell casting system. Currently they all work together as a cohesive whole. If we wanted to change one (say parma) in order to give it a differnt flavor that will have huge ramifications for the rest of the spell casting system.
The set of ablative parma rules I linked to was a perfect example of these interconnections. It assumed a different penetration system than the one in the 5th edition of ars. To use that version of the parma magica without making changes in the rest of the spell casting system to integrate it would be silly. It has already been brought up that the current penetration system, combined with those ablative parma rules, would lead to an undesired tendency to use low level spells to break down a mage's parma. This would not jive with the flavor that we were seeking to introduce with this ablative parma system in the first place. Therefore, if we still feel that this set of parma rules is desirable, we need to change the rules for determining penetration totals. These changes will almost certaingly in turn result in the generic rules for magic resistence and the penetration rules not working properly together. We can see from this example how these changes can be forced to propagate through the system.
This propagation ultimately results from the fact that we are changing one of the underlying design decisions upoon which a number of other decisions were based. This is not to say that this means these changes are bad. They could well lead to a richer game. It does mean that we need to recognize the difference between an essentially cosmetic change, like many of the house rules listed in the house rules thread, and a deeply structural one. The parma magica is in many ways at the heart of the ars magica magic system. Changing it requires a lot of thought.
Edit: Did you mean the attacking spells magnitude equals the defenders parma score? That's a pretty good idea.
Relying on PeVi to wear down Parma , seems to have the same in game ramifications as using ReVi to provide magic resistance.
That ALL Magi will need to put XP in Perdo & Vim , and that specialists will outclass those that don't.
(depending on years out of gauntlet and all that)
If you do not consider HoH:MC to be a Splat Book , then Verditious Elder Runes as a Minor House Mystery
is not too far-fetched for relatively young magi.
Having a Magical Focus as well is not pushing believability.
A Rune for Technique and one for Form combined with an appropriate Focus will give :
one Art x02 and the Lowest Art x03.
altering the example (inset , page 128) to Perdo & Vim
Perdo 08 (x03) + Vim 16 (x02) + 03 Puissant Vim + 03 Intelligence + 06 Magic Theory + 03 Aura
- 05 Shape & Material + 04 Philosophiae + 06 Craft (Metalsmith) + 02 Puissant Craft = 88
Designing the Effect :
Level of Spell (50)* + 10 for 02 Runes inscribed + 10 Unlimited Uses + 18 (Penetration of 36) = 88
*Base 30 + 02 Voice +02 Room
It will take 02 seasons to make this item.
Obviously there are more efficient ways , but i am using a modified book example.
So you will have an Unlimited Use item that casts a Level 50 Wind of Mundane Silence
I don't disagree to the core of your argument. That these rules are important and that they have to mesh. I just dont intend to make those changes in my own game, as I like it the way it is - I'm just playing along to aide you in your ideas.
Indeed I did, I just missed out a very crucial word! In a way it does the same as the abridged breaking down of the rules in the WoMS spells - namely that the level of the spell has to match the Parma Magica (x5), or in other words, the MR minus the Form bonus (or other bonuses). This way you'll have a generel wear on Parmas but without the risk of spamming low-level spells. There's even a game of trying to moderate your spells to what level you expect your opponents defences to be at.
I'd still prefer to keep it as is, or to use the Bonisagus alternative, or to stick to the PeVi spells.
Ravenscroft, while your argument is not unsound it is somewhat moot. What you are attacking is already in the RAW - and in that regard your are complaining about the RAW as is, rather than the thought experiment on making new HR to support an ablative parma.
What your argument in fact does is to motivate for HRing the matter of PeVi, but for other reasons.
I'm not really understanding what is being looked for in a House Rule for Ablative Parma then.
Is Parma still wanted as a mechanic for Magic Resistance?
Why not just use ReVi for Defense and PeVi for Attack?
10 Ranks in Parma at 275 xp is two Art Scores at 17 & 15 (273xp) or 03 Art Scores at 13 (273 xp)