Here's one for ya. I've thought about this, but have not yet implemented or playtested it. I found something vaguely like it in one of the RMSS Companions, but can't remember where. Right now my gaming group is dead, so I don't know when or if I will get the chance to test it, either.
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Riding the Magic, or the Herding Cats Rule
I have watched a man trap a quarter against a wall with the point of a rapier. He does it for practice. I submit that, for fine control of complex forces, such a feat is child's play next to the casting of a spell.
Controlling the forces involved in even a simple spell is like herding cats. And yes, RRs, SF and ESF all address how that affects the caster and the target. But a poorly controlled spell can affect everything in the area, not just the two endpoints of the spell in question. And just because the spell is successful doesn't make it perfectly controlled.
Such is the logic behind Residual Aura, Aura Contamination, and the Herding Cats Rule.
1. Residual Aura - Most spells will leave a "stain" or "stigmatus" in the area they were cast, that will fade slowly. Channeling is Divine, the only imperfections are in the caster himself, so they only leave a stigmatus of the spell's level in a 1'r/lvl sphere for 1 month/lvl. Mentalism inherits the imperfections of its source, but because the source and the caster are one, the understanding of the power flow is quite intimate. Mentalist spells leave a stigmatus of twice the power, size and duration of Channeling spells. Essence power is the most ubiquitous and least understood of the powers, and has the fewest limits. If you do too well at drawing power and too poorly at controlling it, in Channeling you are limited by the Diety's will, in Mentalism by the limits of your mind and body. Potential damage from loss of control of Essence power is limited only by the mass and spin of the world you're on, and by entropy. Essence spells leave 3 times the stigmata of Channeling.
1a) All stigmata of any realm lose 1'r and 1 lvl of power per month. Thus, a 6th lvl Channeling spell leaves a stigmatus of 6th lvl in a 6'r that takes 6 months to fade. A 9th lvl Mentalism spell leaves a stigmatus of 18th lvl in an 18'r that takes a year and a half to fade, as does a 6th lvl Essence spell. A 50th lvl Essence spell leaves a stigmatus that doesn't fade completely for twelve and a half years.
1b) Spells channeled directly from a Diety leave no stigmata unless the Diety so chooses. Arcane spells will leave stigmata comparable to Mentalism.
1c) Note that none of this matters unless you are casting spells in the same place every day, as in your home or shop. For game purposes, roll d100 (high open-ended)/2 at the beginning of spellcasting in a given area to find out the beginning residual aura. Modify by -30 for rural areas, -10 for small towns, +10 for ruins/dungeons/cities. Results below 0=0.
2. Aura Contamination - Auras interact with one another, sometimes resulting in things none of the original casters intended, or indeed imagined. These are called "improbabilities" or "Fortean effects" or other names. It is randomized magic flow, with magic's ability to redefine local reality, but without control or purpose.
3. The Herding Cats Rule - Any time a spell is cast, there is a chance equal to (100 - BAR/EAR) + (residual aura/2) of an improbability/Fortean effect. The GM rolls d100 open-ended. If (100 - BAR/EAR) + (residual aura/2) + GM d100 = 101 - 200, there is a harmless minor effect within 1'/spell lvl of the caster. 201 - 300 = a medium effect, possibly harmful, possibly not. 301 - 400 = major improbability/Fortean effect, distinct possibility of seriously harmful results. 401 - 500 = Extreme Forean effects, harmful and possibly lethal. 501+ = Reality storm/Everything within 1'/spell lvl of the caster randomly gated to another plane of existence/something else both weird and catastrophic occurs.
3a) Note that only extremely powerful spells or exploding dice will get you into severe trouble *right away*, but the chances of something really BAD happening go up quickly. Since the stigmatus of the spell you are casting will be figured into the next spell's residual, a party with two or more spellcasters will want to keep their battles on the move. If a Mentalist casts a 5th lvl spell and a Magician casts a 6th lvl spell, the next round's residual just went up 14% ([(5*2)+(6*3)]/2). A protracted battle could get really weird, and seige would have a whole new horror.
3b) Note that BAR/EAR rolls include all bonuses and penalties, and a roll above 100 can reduce or eliminate the stigmatus of its casting. A 10th lvl spell cast with a BAR/EAR of 131 or better leaves no stigmatus (111 or better for Channeling, 121 or better for Mentalism/Arcane). An exremely high BAR/EAR temporarily reduces the chance of unwanted effects, weaving the residual aura itself into the new pattern.
Example: Julie Anne Freys (Fighter), Duncan Boxer (Warrior Monk), Flaude Thierry (Magician), and Cy Omega (Mentalist) are fighting giants. They are all 10th lvl, and they encountered the giants in the ruins of an ancient city. Background aura is (22/2)+10=21. Cy wins initiative, casts a 5th lvl spell, rolls 43 with a bonus of 36 so 43+36=79. 100-79=21, 21+21=42% chance of an improbability. GM rolls d100(43), 43+42=85, nothing happens. Flaude fires a lvl 6 Elemental attack with a +44 Directed Spell bonus. Roll 29+44=73, 100-73=27, 27+21+5(1/2 the lvl 10 stigmatus left by Cy's spell)=53% chance of random effect. Gm rolls 59+53=112 and a small bird flies out of Cy's beard. It had to be Cy or Flaude because Julie and Duncan are in melee range of giants, and thus are obviously not within 18'(lvl 6 spell*3 for Essence) of Flaude.
Round 2, 26+9=35% chance of effects. Flaude wins initiative, fires another elemental attack, this time 94+44=138. 35-38=-3% chance of effects. No effect, no roll needed. Cy casts another 5th lvl spell, 50+36=86. 100-86=14, 14+35+0 (note that Flaude's elemental attack left no stigmatus)=49. Gm rolls 66+49=115, and all flying insects within 5' of him turn into brass sequins and fall to the ground.
Round 3, Flaude wins initiative again, gets overconfident and fires a lvl 10 attack with a bonus of +15. 11+15=26. 100-26=74, 74+35+5=114% chance of something strange happening. GM rolls 98+96+93+114=401. Not only did he *badly* miss, but his sloppy control accidentally causes 36d10 live piranhas to rain on an 18'r centering on him. Cy decides instead of attacking this round, he's going to move farther away. If Julie or Duncan happen to notice what's going on, they'll have to make SD rolls to keep from breaking up laughing. Good thing their backs are turned.
It sounds complex, but it isn't. Background aura changes by half the stigmatus of the last spell cast, just keep adding it on. GM rolls against background + however much the casting missed 100 by, so the more skilled you are the less you have to worry. But a spell failure is not just your problem, it's everyone's.
