Houserule chaotic magic

Chaotic Magic virtue (Alteration) [ArM5 p.52]
Your magic is very wild. When you cast a spontaneous spell, don’t botch and your casting total is high enough for the intended effect, you must also make a chaotic magic roll by rolling a simple die (SD). If you don’t roll high enough you simply fail to channel enough magic energy and nothing else happens:

Chaotic magic roll (SD):
-9< you cast a completely different spell of same TeFo combination at double effect level (SG decision)
-8 the spell does (much) less OR the opposite of what it should (like healing, instead of wounding)
-5 to -7 you have to invest a pawn of vis OR have –3 penalty to any casting total for the rest of the scene/situation
-1 to -4 –1d10 penetration OR loose 1 extra fatigue level (SG decision)
0 one spell’s parameter (range, duration, target/size) changes by 1 step to the detriment of the caster (SG decision)

1 roll a second (SD + pawn of raw vis used) and subtract the value from the first roll
2 unpleasant cosmetic side effects
3-8 the spell works as intended.
9 beneficial cosmetic side effects
10 roll a second (SD + pawn of raw vis used) and add the value to the first roll

11 one spell’s parameter (range, duration, target/size) changes by 1 step to the benefit of the caster (PC decision)
12-15 +1d10 penetration OR loose no fatigue level(s)
16-18 you may invest a pawn of vis AND have +3 bonus to any casting total for the rest of the scene/situation
19 the spell does (much) more than it should (like double damage) to the benefit of the caster (PC decision)
20< you (have to) cast an additional spell of same TeFo combination and at same level of effect for free