I swear on all things unholy that the descriptions in Ars Magica 5th edition on how Magic Resistance (MR) and Penetration work are confusing. As a roleplayer’s PSA here is my breakdown of how it works, with a worked example that explores some of the edge cases you’ll encounter.
(I’ve checked with the gurus in the Ars Magica official forums, so the worked example below should be accurate.)
So how does MR and Penetration work in Ars Magica?
The simple explanation is that the penetration total for a spell must be higher than the target’s magic resistance. If they have no MR, then the spell effects are resolved as normal. So the Penetration total must exceed the MR.
As a basic principal the more raw power the Magus has to cast the spell the better, however that power is used for the generation of the spell effect and also for determining if it can pierce the Magic Resistance of the target. Essentially the more power the Magus uses in generating the spell, the less power they have to defeat any MR they might encounter.
That might seem odd, but it makes sense when you consider that most mundane targets (most normal humans) have no magic resistance at all. So against those targets the caster can opt to use their best very powerful spell (such as an instant death spell) instead of a far weaker spell that might just hurt or paralyse the target. The lesser powered spell might harm a creature with MR, as more of the remaining potential energy is being used to penetrate the creature’s resistance.
However almost all of the supernatural creatures, Magi, and some holy men do have magic resistance from various sources, and their MR must be overcome before considering the effect the spell might have. Thus the Ars Magica rules were updated in 5th edition to demonstrate that MR is valuable and something that Magi must consider when fighting with magical enemies.
So the penetration is the remainder when spell’s power level is subtracted from the power the magus generates. This means that sometimes the penetration value (the remainder) is less the magic resistance of the target, and therefore the spell is repelled. Sometimes the remainder is negative, which means that while the spell was cast successfully, the spell could only affect creates that have no magic resistance at all. i.e. It would only affect mundanes.
How does it work mechanically?
As a start there are a few formulas, there are three types of Casting Totals (CTs), and a few scores from the character sheet that might be needed. In the example below the Formulaic Casting Total is used, but either of the Spontaneous totals could also be substituted if the characters chose to cast spontaneously.
The goal of the caster is to exceed the Magic Resistance of the target with their Penetration total. If the Pen total equals but does not exceed the MR, the defender is not affected and the spell is repelled.
Formulaic CT = Stam + Tech + Form + Aura + d10 + bonus Spontaneous (with Fatigue loss) CT = (Formulaic CT) / 2 Spontaneous (with no Fatigue loss) CT = (Formulaic CT) / 5 Magic Resistance = Attacking Form + Parma Magica * 5 + Aura Pen Total = CT - Spell level + (Pen Score + Pen Bonus) * Multiplier 1
You also need to know the spells being cast, and the power of each caster. As a worked example lets use the following two characters:
Xavis, Magus ex Flambeau Attributes = Stam 3 Abilities = Penetration 2, Parma Magica 3 Arts = Creo 9, Ignem 11, Terram 0 Spell = Arc of Fiery Ribbons, CrIg25 (ArM p140) Corisantu, Magus ex Tremere Attributes = Stam 3 Abilities = Penetration 2, Parma Magica 3 Arts = Muto 9, Terram 11, Rego 9, Ignem 0 Spell = The Crystal Dart, MuTe10(Re) (ArM p154)
What do we expect happens?
The Magi roll initiative, cast decide to cast their formulaically, and to keep it simple we say that Xavis wins and gets to cast first.
Xavis attempts to casts Arc of Fiery Ribbons, rolling 9 on his casting roll. This gives him a Casting Total of 32 (Roll 9 + Stam 3 + Creo 9 + Ignem 11 + Aura 0). His spell is cast successfully, and costs no Fatigue.
Xavis then determines the Penetration Total is equal to 9 (CT 32 – Spell Level 25 + Penetration Bonus 2). The Pen bonus is 2 because the of the Base ability 2, multiplied by 1 due to having no additional modifiers (see Arm p84 for the possible modifiers).
Corisantu has a Magic Resistance of 15, which is Corsantu’s Ignem 0 + (Parma Magica 3 * 5). Pen Total 9 vs MR 15.
When the Penetration Total is compared to Magic Resistance it is easy to see that Xavis’ spell did not penetrate Corsantu’s MR.
Now Corisantu retaliates with her own spell, The Crystal Dart, rolling 5. This gives her a Casting Total of 28 (Roll 5 + Stam 3 + Lower of Muto or Rego 9 + Terram 11 + Aura 0). Her spell is cast successfully, and costs no Fatigue.
Corisantu then determines the Penetration Total is equal to 20 (CT 28 – Spell Level 10 + Penetration Bonus 2). The Pen bonus is 2 because the of the Base ability 2, multiplied by 1 due to having no additional modifiers.
Xavis has a Magic Resistance of 15, which is Corsantu’s Terram 0 + (Parma Magica 3 * 5). Pen Total 20 vs MR 15.
When the Penetration Total is compared to Magic Resistance it is easy to see that Corsantu’ spell penetrated Xavis’s MR; and Xavis is now inflicted with d10+10 damage. This demonstrates that when two roughly equal power Magi fight the more Penetration the better, and it is easier to get higher penetration when casting lower level spells.
Its likely that if the combat continued like this Xavis would be badly injured very quickly, and Corisantu would be basically unscathed.
The combat round above would be very different if Xavis had a few tweaks to his character creation, to further specalise in defeating MR.
If Xavis had an increased his Penetration score to 4, and then added a specalisation in Penetration for Ignem spells (which adds one to the effective level of the penetration ability for Ignem), then his effective base Penetration ability for that spell would be 5.
This still only gives a Penetration Total is equal to 12 (CT 32 – Spell Level 25 + Penetration Bonus 5). Not enough to defeat MR 15, but darn close.
If Xavis also had spent XP mastering the spell Arc of Fiery Ribbons twice (a fair amount of XP but well worth it for spells often used), and selected Penentration as a mastery talent (ArM p86), then each point of mastery adds to the Penetration score, and also adds to the base casting total. Now the CT has increased to 34 and the Penetration bonus is now 7, (Pen Ability (4+1+2) * 1). i.e. The pen bonus of becomes (7 * 1) = 7.
This gives Xavis a Penetration Total equal to 16 (CT 34 – Spell Level 25 + Penetration Bonus 7). This defeats the MR 15, and Xavis now gets to roll the d10+10 in damage that the Arc of Fiery Ribbons inflicts.
Xavis, Magus ex Flambeau (updated) Attributes = Stam 3 Abilities = Penetration (Ignem) 4, Parma Magica 3 Arts = Creo 9, Ignem 11, Terram 0 Spell = Arc of Fiery Ribbons, CrIg25 (ArM p140) - AoFR Mastery (2): Penentration and Fast Casting
Through use of Spell Mastery and Ability Specialisation an advantage can be realised in defeating Magic Resistance. Its worth stating that Corisantu could also specalise and master her spells, and thereby boost her penetration totals just as easily, and either Magus could also also have added a specialisation to their Parma Magica too, if it was lucky enough to match.
Important note – You’ll notice that the Spontaneous Casting Totals are far lower than Forumulaic, as they are divided by either 2 or 5. That is correct and has a deliberate and direct negative impact on the penetration of the spells as well; as the Casting Total will be so much lower.
Don’t forget – Fast-casting of a mastered Formulaic spell suffers a -10 modifier to the casting total, which in turn is also a penalty to the Penetration score. That keeps the theme of splitting power between the spell and the penetration consistent.
And Auras – as noted in the first formula breakdown, Aura is added to to the casting total, but it is also added to MR.
Happy casting folks, Ironbound.