Reconstructing Ars Magica: the basics

This post gives new magic rules inspired by Ars Magica. It's written for my own fantasy heartbreaker, but it should be compatible with most or all OSR games. Try using this in place of your Magic-User's normal magic rules, or add these as variant casting and spell-learning rules for a different casting class.

These rules offer supreme flexibility and extensibility for a sort of magic grounded in magical traditions. I provide links to the traditions at the bottom of the post.

These rules also model spell research, including the creation of brand new spells, better than any I have seen in any OSR game.

What a spell is

A spell is a combination of a noun and verb, spoken out loud, plus some gestures and effort. Your specific magical tradition is going to give you the specific list of nouns and verbs that can combine to form a spell.

The same noun-verb combination can be used to cast a bunch of different spells; the noun and the verb just give the basic family of effects. For instance, "Give blessing" might give some number of bonus dice on a task, or it might grant you a (temporary) skill or ability increase.

Within the noun-verb combination, we construct individual spells by varying their specific effect, range, duration, and target. Thus "Give blessing" could be a spell that gives two bonus dice to the caster for five minutes, or it could give five dozen people within earshot the ability to speak French for a month.

We'll say the spell has some magnitude based on its specific effect, range, duration, and target. Spell with a greater effect, range, and duration, and more targets, have a larger magnitude, all else being equal. A spell with a larger magnitude is more difficult to cast than a spell with a smaller magnitude, all else being equal. The default spell has a negligible range, only affecting your body or the things you carry. Its duration is immediate, less than a second. And its target is just one person, probably you.

The effects, ranges, duration, and targets available to you, and their, uh, effects on spell magnitude, all depend on the individual magical tradition.

You might figure out a spell in advance, as a downtime activity. You could learn it from a book or mentor, or you could invent it yourself. If you've figured out a spell in advance, we'll say you know the spell's formula. On the other hand, you might just wing the spell, not knowing anything about the spell in advance. Then we'll say you cast the spell spontaneously.

How to cast a spell

Casting a spell as a formula is a task with a difficulty of 1/6, a period of ~10 seconds (1 combat round), requiring a number of successes equal to its magnitude, with dice equal to your casting level. It requires some skill in each of the noun, verb, and specific spell to attempt it. Get bonus dice from your Body/Constitution, and by spending 1 HP/die on the casting attempt. Adjust TN based on your relevant skills.

When you cast the spell, you use a firm voice and bold gestures. It's obvious to everyone that you're doing magic.

If you are distracted while casting, make a concentration check (3/6, bonus/penalty dice from Mind); on a success, take a penalty die to the casting roll, and on a failure, simply fail the casting roll. If you are actually hindered while attempting to cast (for instance, if someone is attacking you) you automatically fail the roll.

Casting a spell spontaneously is similar. Differences: it requires twice as many degrees of success for the same magnitude spell; you get bonus dice from your Mind/Intelligence, rather than your Body/Constitution; and, naturally, you can't and don't have any skill in the spell itself. (If you did, you would be casting the spell as a formula.)

That was really dense! Below we'll break it down and discuss some tricky details.

You probably don't want to actually read through all those links, so let's reiterate the basics of task resolution and skills.

First, skills. A skill is something you can know and improve at. The obvious skills for spellcasting are the noun, the verb, and the specific spell. A skill is rated according to its quality: some practice, basic competence, proficiency, mastery.

Now, tasks. [To be written. Fuck me, I've re-summarized the task resolution rolls a million times in the past month, I'm too tired to do it again now.]

Back to casting.

For every relevant casting skill at proficient or higher, +1 TN. For every relevant casting skill at mastery, an additional +1 TN (for a total of +2 from the skill). So if you've mastered the noun, verb, and specific spell, you get a total of +6 TN, meaning the spell has a difficulty of 7/8. (Recall that if you get above (D-1)/D, you bump up the die size and increase the TN accordingly; +1 TN to 7/8 is 9/10, then 11/12, then 19/20... Similarly, for lowering the TN below 1/D; 1/6 to 1/8 to 1/10...)

If you get enough degrees of success (equal to the magnitude of the spell), the spell goes off and the casting ends. If you get extra degrees of success, increase the penetration of the spell (in other words, it goes through the target's defenses somewhat. Dragonfire is the paradigmatic penetrating effect; on a save, it still does half damage.).

If you get some degrees of success, but not enough to cast the spell, you can take an extra casting period to get a bonus die, or you can let your accumulated degrees of success fizzle. Make a save or a Mind check or similar or distribute the fizzled degrees of success as damage, 1 HP per degree, in your immediate vicinity.

If you don't get any degrees of success, you've botched the casting. Prepare for the worst, probably a roll on your most dreaded magical mishaps table. Alternatively, suffer a reversed or inverted version of the spell.

How to improve your chances at casting a spell

Spend downtime to learn a spell as a formula. Immediately cuts the number of successes required in half.

You get one base die per caster level. Level up and you'll have a better chance of casting the spell.

You get one bonus die per point of HP spent on the casting roll. Sacrifice yourself and you'll have a better chance of casting.

Use some magical material, which is destroyed in the casting, for 1 bonus die per item. For instance, a personal effect of the victim for a voodoo-type spell, a magical item, alchemical ingredients, the parts of a highly magical creature. Obviously the material has to be relevant to the spell, you can't just use any old magical thing.

Finally, use downtime to improve your skill in the noun, verb, and specific spell.

Some metamagical skills

Here are some suggestions for skills beyond the noun, verb, and specific spell that can modify your casting. They might mess with the target number, add bonus or penalty dice, or change the circumstances of your casting, figure it out.

You might want to specify which magical tradition has access to which metamagical skills, or if some metamagic is easier for them than others. Or just leave it all open.

  • Fast casting, cut the period of the spell to "immediate"
  • Subtle casting, don't use bold gestures and a clear voice
  • Ritual magic, increase the period and cast in a ritual setting but modify TN
  • Magic theory, makes learning/creating new spells easier?
  • Alchemy, create magical material for casting, plus potions, salves, incenses
  • Astrology, get some bonus to casting when the stars are right (lmao who will bother modeling the movements of the stars? Min-maxers I guess)
  • Geomancy, bonus to spells in certain locations affine with the spell
  • Blood magic, get bonus dice from sacrificing others, 1 die per level
  • Enchantment, store the spell in some item or make a D&D-style magic item
  • Spell scribing, make books and scrolls for others to read
  • Intuitive magician, increase a magical skill when you see somebody else use it
  • Focused caster, resist interruptions to casting
  • Careful caster, mitigate risks from spell successes fizzling or spells botching
  • Flexible caster, when you don't have enough degrees of success you can cast a lesser version of the spell instead

What skills and spells do I start with?

Damn that's a tough one. Ars Magica makes it a whole sub-game, where you chart out the childhood and apprenticeship of your character. Here's a quick attempt. I will probably revise it later after we've played for a while.

If your character is an apprentice to an experienced magician, start with a bit of experience (not much) in several of their skills, and three or four low-magnitude spells.

If your character is a lone adept, go nuts, pick whatever zany option you want, but very few of them.

A high-level magician is gonna know like 30+ spells and has basic competence in half or more of the nouns and verbs, plus mastery of a few.

How do I get more spells and skills?

Learn spells from teachers or books. Requires some skill in the noun and verb. Let's say you need 10 * magnitude degrees of success on a learning check with a task period of 1 day. Bonus dice from level, a really good book or great teacher, decreased TN if the spell is out of your wheelhouse. Start with some practice in the spell.

Invent spells in a laboratory. Requires some skill in the noun and verb. Let's say 10 * magnitude^2 degrees of success, task period 1 day. Bonus dice from level, assistants, and a great lab, reduced TN from skills. Obviously this required magical material and a lot of money. Start with basic competence in the spell.

Ars Magica limits the magnitude of the spell you can learn or invent. Maybe we should, too; let's say it's equal to your level + 1, +1/+2 for proficiency/mastery in magical theory, +1/+2 for proficiency/mastery each in the relevant noun and verb. I haven't thought too much about this though, it seems likely that proficiency/mastery should increase the magnitude by a lot more.

Study with a teacher or in a laboratory to increase skills. Not sure about the task period; since magic isn't real I can't just look up how much time it takes to learn things. (Maybe this should vary (greatly) by campaign; maybe we want a campaign where all the magicians become superheroes immediately and fuck around in the cosmos, or maybe we want a campaign where you have to grind for years to learn a single basic spell.)

If you use the percentile rules for skills (which I am moving away from, probably too granular for my game, since I abstract away so much downtime) you probably get +Level%, spread out among your magic skills, every session.

Ok, that's it for the basics lmao.

Now you gotta read up on specific magical traditions to get their lists of nouns and verbs, and to see if they have any wrinkles on enchanting.

Traditions I've written up:


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