If you want a flexible but dirt-simple magic system, let’s hack it this way:
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Add a new skill called Arcane Mysteries (or something similar), OR (and I like this better) just use Lore Mouse (in Realm Guard, this would be Lore Master). If you create a new skill for this, then during character generation it is available on the same lists as Lore Mouse, and none of the others. When used for MAGIC, it can be helped by Lore Mouse, appropriate wises, and sometimes Science and Healing.
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When used to do MAGIC, the skill can (like Science and Militarist) be used to affect opponents normally ‘too big’ to be affected by a single character.
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The skill has its own specific uses, but in this hack it can also be used in lieu of any other skill in the game, without requiring the trappings of said skill. This use is the invocation of magic.
** When doing such a swap, narration of the effects describes how character magically, say: finds a path, changes the weather, crosses a large body of water, and so forth. It’s important to know what skill is being supplanted by the magic, so you know what kind of effect the magic is supplying.
** Such a power has a cost. Even if the character succeeds in a test where this skill replaced another, they will still be, at the least, Tired. Repeated use when the character is already Tired will inflict increasingly more detrimental Conditions.
** If the character fails such a check, they will suffer an Arcane Backlash Twist. Use your imagination.
** A character can do this ‘skill swap’ during a Conflict for as many rolls as they like – they only deal with the consequences of passed and/or failed checks after the Conflict, but suffer such consequences above and beyond the ‘main’ results of the Conflict. Keep in mind, however, that they may actually end up dealing with the consequences of both successful AND failed magic checks after a Conflict, if they used it multiple times, and got both results.
***Note: While they might face both success and failure magic test consequences after a long Conflict, they only face one of each, maximum; they won’t get hammered with three ‘success’ consequences after a single conflict, just because they used this option three times.
That pretty much gives you a spell system right there. Swap in the Arcane Mysteries to do anything that you could conceivably do with another skill, without having the equipment for that skill on hand, narrate appropriately, and off you go.