Illusions

Illusions (and charm spells) have dropped out of modern editions of D&D.  They're hard to adjudicate, and so the emergent practice has been to set them aside in favor of spells that have mathematical effects.  Few things derail the tension of a combat situation more than a full five-minute argument over how an illusion does or should work, possibly including book references that all add up to "DM discretion" anyway. 

But I don't think it has to be that way.  I think it's possible to write rules for illusions, and spell descriptions for illusion spells that clarify "this illusion spell ALWAYS does these things, it NEVER does these things, and it does THIS or not based on the die roll."   (I'm trying to be edition- or system-neutral, but I make no promises.  The reader is a big boy or girl and is capable of swapping out Intelligence saves or checks for Will saves, or whatever OSR save you think fits)

So we need to pick design targets, so that we have some guidelines for power levels.  This is one advantage of only using lower-level spells (relative to D&D).  Capping at level 5 means you have cantrips and 1st-level spells, the bread-and-butter of novice spellcasters; 2nd level spells as the "big guns" rolled out by veteran wizards, and 3rd level spells, the apex of what a caster can do on a given day. 

One design rule-of-thumb is that an illusionary spell should be able to mimic a spell one level higher, with the limits of illusion spells.  So an illusionary fireball slots in as a 2nd level spell, Will save negates, does half the damage (nonlethal) on a failed save.  Illusionary monster summoning gets you a slightly better monster than a same level summon monster, but does no damage on a successful Will save, has fewer hit points etc (it's made of less real material, like comparing a blimp to an airplane). 

I'm looking at how we scale illusion spells.  Published illusion spells scale by size, by duration (1 minute, 10 minutes, Concentration(Y/N)) , and by level of detail (static image only, visual image that can move, visual image backed by small sound, smell, temperature details).  I think a more useful approach to scaling might be Belief.  Instead of having the same saving throw rules-and-mechanism for all illusions, we could set up a scale from "illusion disappears when touched" to "failed save means target believes the illusion permanently" with gradations in between.

And I've worked up some "theory of magic" to make illusions work--illusionists are not theoreticians (evokers or transmuters might spin grand theories about the fundamental nature of this or that, but not illusionists) but engineers, jerry-rigging together "good enough" combinations of weak evocations and partial-conjurings duct-taped together with "cerebromancy" to fill in the gaps between a stick figure person and a fully realized illusionary warrior.