RM2E compendium 5 has an additional Druid Spell list called Nature's Summons (pg 55 RMC5). The spell list doesn't concern me, but the disclaimer prior is difficult to actually understand. As a note it says that spell levels 10-16> take at least 6-12 hrs to cast but only have a 2% chance per level of the caster to actually work and product a desired effect. My player looks at this and basically says it's useless to try and cast any spell in this range, because even if everything is done correctly and the spell goes off without any problems, there is a chance it won't work, and might even backfire. With these kinds of notes it makes casting of some spells restrictive, and really useless.
So do I take it to mean that even if the spell is cast with no problems but the required percentage chance the spell still wont work is not made, that the spell is a bust? Or is this like a lingering effect that has potential to still take effect over time? I think that aspect would be a house rule, but just making sure the note is understood is my primary focus here.
Any help would be great.