About a year ago, Wombat Warlord and I spent a good bit of time working out ideas to add a robust and flexible magic system to Song of Ice and Fire Roleplaying, because we liked the core of the system enough to adapt it to other fantasy settings. In particular, WW was preparing for the Birthright game he now runs. He's modified his system further since I last did any work with it, so this post won't be offering direct commentary on what he's using, only presenting what we did at the time and improvements I'd still want to make.
As a quick summary, SIFRP has 19 separate ability scores, because it doesn't separate "ability scores" and "skills" the way D&D does. Where D&D treats Charisma as a "ruling stat" over Bluff, Diplomacy, Streetwise, and so on, SIFRP has Deception, Persuasion, Status, and the like as equally available abilities. Each ability is rated in d6s, and an average level of skill in any ability is two dice. Target difficulties scale so that average tasks are within easy reach for average people, while highly skilled characters will be counting up degrees of success on average tasks. Players gain additional dice from skill specialties, rolling more dice based on the degree of the specialty, but only keeping a number of dice equal to their ability score. The system does a solid job of making all of these abilities useful.
From these ability scores, there are derived stats: Intrigue Defense, Composure, Combat Defense, Health, and Injuries. Intrigue Defense is a difficulty rating for attacks from other characters that would cause you to lose Composure, while Composure is your health pool for social attacks, and Disposition is a variable armor value that applies to intrigues. Combat Defense is your ability to avoid getting hit by an attack; heavy armor reduces this number, but soaks damage. Health is a very small number, but it's expected that characters will take Injuries (which are relatively short term) and Wounds (which are long-term) in a prolonged combat.
Characters also buy Qualities, which are more-or-less feats, including feat-like progression. Through Qualities, fighting styles become increasingly like prestige classes, with situational bonuses and exceptions to the rules. SIFRP's existing mystical abilities - green dreams - are also handled through Qualities.
Two New Abilities
One thing clearly absent from the SIFRP ability list is any magic-focused stat, though Animal Handling applies to green dreams. There are cogent arguments to be made for Knowledge, Cunning, Will, and (for some settings) Language, but adding magic as a primary function to any of these stats would be severely unbalancing. Therefore we decided to add Wizardry for arcane magic and Piety for divine magic. Unlike normal abilities, however, these abilities must be unlocked with the purchase of a Quality, and then must be purchased up from a base score of zero dice.
There are two different Qualities that a character can use to get some benefit from Wizardry: Hedge Wizardry and True Wizardry. True Wizardry carries the additional prerequisite of any bloodline Quality, as Birthright's more powerful magic is available only to those with a mystical bloodline. A similar pair of options are presented to character pursuing Piety: Touch of the Divine and Power of the Blood.
Wombat Warlord further created specialties for Wizardry and Piety, as exist in other abilities. These specialties represent core spell types, with partial overlap between arcane and divine magic. Each spell type can work with a limited number of spell descriptors, much like the Qualities possessed by weaponry; each spell, then, is in a sense a new weapon assembled ad hoc.
A spell's casting difficulty, which is for most spells also its chance to take effect, uses the caster's Wizardry or Piety, paralleling Fighting or Marksmanship. The spell's potency (damage healed/dealt, and so on) is Cunning for wizards and Will for priests, paralleling Athletics or Agility. The caster's power reserve (currency spent for casting) is Composure (derived from Will) for wizards, which came from the idea that wizards are holding complex mystical geometry and the like in their minds and are mentally drained by the experience. Priests use Health (derived from Endurance), because I interpreted Birthright divine magic as all about the power of blood (even for the Good religions), and I liked the idea of making all priests heavily scarred but unbelievably tough.
Some Alternate Ideas
WW has wizards take Injuries and Wounds when they run out of Composure, so that the wizard can take more lasting consequences when scraping the bottom of the barrel for power. I came up with a variant idea for that, such that the more severe of the two (parallel to Wounds and lasting at least a week) are Curses. The story that backfills from that is that the powers of the land, from which the wizard draws power, are angered and exact their wrath in the form of a debilitating Curse. The lesser form (parallel to Injuries and passing in a few days) is simply be Strain. When healing Strain and Curses, Will tests replace Endurance. I'm not sure if there's a skill that someone else can use to help you purge Strain and Curses, but Persuasion sets up an interesting story possibility.
The second addition I'd want to make to WW's system is a sense that spellcasters have to learn spells in advance rather than casting spontaneously, which also means that there are pre-defined and inflexible spell parameters. This is just a matter of my personal taste in magic systems, but I love it when characters need to research new spells and/or capture spellbooks and sacred scrolls from enemies. Therefore, wizards use Knowledge to comprehend arcane mumbo-jumbo. After some debate, I've settled on recommending Status for priests to learn new spells. I feel that the religions of BR are more about organized than they are about religion, and the goal of PC priests in domain play is to establish a widespread sect of their religion under their personal control; I think, therefore, that the gods would grant the favor of new spell knowledge to priests based on their place in that hierarchy. (One could just as readily attach Knowledge and Status to spell preparation rather than spell knowledge, depending on how you want all of this to work.)
But I didn't stop there. I liked my little chart, with its Arcane and Divine columns and its rows of Spell Knowledge, Accuracy, Potency, and Reserve. So I added two more columns, with help from Kainenchen. These magical traditions could be shoehorned into BR, but they might fit better in some completely separate setting.
Spell Knowledge: Awareness
Accuracy: Shamanism (new ability)
Power Reserve: Will (or Endurance, if you want blood-sacrifice druids)
Spell Knowledge: Language
Accuracy: Goetia (i.e., Demonology - new ability)
Power Reserve: Will (or, again, Endurance... this doesn't bear further explanation)
I particularly imagine Goetic Magic involving lots of Intrigue Challenges with summoned demons.
And - of course! how could it have been otherwise? - now that I've given it this much thought, I really want to write a new setting and run stuff using this. Maybe some future posts will include details of arcane, divine, or other spells.
PS. Wombat Warlord, I'm totally calling you out to post some of your casting rules in your blog!
PPS. Wombat Warlord has replied, and I link it here for future reference.