Reconstructing Ars Magica in D&D: motivations and dials

The magic rules in D&D are deficient from a literary perspective, and played out from a wargaming perspective. (To be clear, I think Vance is great, but D&D's "Vancian" magic is not particularly Vancian.) So I'm looking for replacements for my fantasy heartbreaker. Most likely I'll try to assemble lots of different rules for magic. We'll choose whatever combination of rules fits the setting for an individual game. I'll make no attempt to harmonize or balance them; we'll simply see how they interact.

A set of magic rules that come highly recommended

I skimmed Ars Magica once. The magic rules seem really cool, though I have little interest in the game itself. So let's try reconstructing it for the heartbreaker.

Here are some salient facts about magic in Ars Magica:

  • Spells have levels/magnitudes. There are guidelines in the book on making new spells at a given magnitude, or finding the magnitude of a given spell. Must review these.
  • The magnitudes are usually between 1 and 15, or at least I didn't see any higher magnitudes.
  • Magnitude = level/5.
  • A spell includes a sentence with a noun and a verb. There are 5 verbs and 10 nouns.
  • Maga master the nouns and verbs separately.
  • Spells can be cast as formulae, with obvious reproducible effects, or spontaneously, and much weaker.
  • Formula spells require rituals, time and resources. And you have to know or develop the formula for the spell. They use stamina. Isn't that odd!
  • Spontaneous spells require intelligence.
  • Spontaneous spells are 1/5 to 1/2 as strong as ritual spells.
  • When you get better at casting a formula spell, you need less and less stuff to cast the spell; from a lab, materials, and a ritual, down to just slight hand motions and murmurs.
  • Maga can spend fatigue to do better on the casting roll; slightly better on formula spells, much better on spontaneous spells (from 1/5 to 1/2 the strength of a ritual spell).
  • Extra success on the casting roll increases the "penetration" of the spell against magical defenses.
  • A starting maga is going to cast a couple spells of magnitude 5 about half the time, maybe all the time with a fatigue expenditure.
  • You can do really well on a casting roll, or really poorly, beyond the simple 1 to 10 range of 1d10.

Here are things we know about characters in the heartbreaker:

  • They have a class, magician.
  • They have a level, which hopefully increases in play.
  • They have stats, including Mind and Body.
  • They have skills, and skills can be about anything.
  • They have resources and downtime.
  • They start a lot weaker than an AM magus; maga seem like decent challenges for Conan, who is probably around levels 4 to 6 in the heartbreaker

Here's what I know about task resolution in Ars Magica:

  • Roll a d10 plus a bunch of numbers from your ability score and skills

Here's what I know about task resolution in the heartbreaker:

  • Tasks have a period (the time it takes to do them), a base difficulty (usually x/6), and a number of successes required to go off (usually 1 for normal tasks, maybe a lot more for spells)
  • Tasks get bonus dice from skills and levels in relevant classes (for instance attacking is a task that gets a bonus die from each level for a fighter)
  • Tasks can require specific materials

Rules for research should port over neatly. The big thing is going to be converting the casting roll. In Ars Magica, maga have scores of (I think) 1 to 20 in each noun and verb, and 0 to 5 in each stat. You cast by rolling d10 + noun + verb + stat > spell level. In the heartbreaker, by contrast, you roll a pool of dice and count up successes.

So here's my draft of formula spell rules.

The base difficulty of casting a spell is 1/6. +1/+2 TN for for mastery of the noun, +1/+2 TN for mastery of the verb, +1/+2 TN for mastery of the specific spell.

You need 1 degree of success per spell magnitude. +1 bonus die per magician level. +1 bonus die for every HP spent on the roll. +1 bonus die for every extra component: magical materials, rituals, labs, blah blah, whatever isn't required by your mastery of the spell but you bring to the table anyway. 1 penalty die for every component missing.

These rules benefit from a more dramatic interpretation of degrees of success/failure, in which your degree of success is your number of hits, and your degree of failure is your number of misses, no matter if you're using bonus or penalty dice. Thus a failure with many bonus dice is going to hurt, a lot!

Compared to actual Ars Magica, these rules will produce a more generalist magician with much less power. That's fine by me!

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