Situation: I am a wizard with Shadow Evocation. I`ve readied an action for Counterspells. I`ve succeded at Spellcraft check to identify spell being cast (Fireball).
Question: Can I use Shadow Evocation (Fireball) to counterspell regular Fireball? If I can, should opponent throw 20% chance to disbelieve?
Hmm... I've never seen this come up before. Upon a close reading of the relevant rules I would say no; a Shadow Evocation spell is not actually a Fireball spell, it's just duplicating the effect of the Fireball spell. Therefor it cannot be used to counterspell an actual fireball (and, by extension, a fireball could not counterspell a Shadow Evocation used to create a Fireball). However, the rules don't state firmly and this is a bit of an edge-case since you are still casting an illusory version of Fireball in this instance.
I would say ask your GM. If they rule that it works then there is no chance to disbelieve; the actual spell effect of the Shadow Evocation doesn't come into effect (it just neutralizes their fireball) so there's nothing to disbelieve.
Alternaticely you could give the enemy the chance to disbelieve, and if they do the counterspell fails. The disbelief is the limiting factor for an otherwise extremely versatile spell, so keeping that as the limiting factor here might help balance it.
Of course counterspelling almost never sees any use, so maybe allowing a little more creative thinking like this will get your party to diversify their tactics
(PS I think the RAW answer is: "No, Shadow Evocation doesn't counter Fireball." I was just trying to think how it could work.)
I would say that the fairest ruling would be that it has only a 20% chance to dispel/counter a spell (in the case of shadow evocation, it would be higher for greater shadow evocation) I wouldn't give the opposing caster a check, since you aren't affecting them. I consider a counter or dispel use to be a special non-damaging effect or use.
If the disbelieved attack has a special effect other than damage, that effect is one-fifth as strong (if applicable) or only 20% likely to occur.
That just seems like the safest, fairest way to go. Obviously in case of specifically affecting a creature, like removing their haste with some kind of shadow slow, you might allow a save (but even on a success that doesn't stop it from working, would just have a 20% chance to do so, as above).
Shadow evocation does not actually create a fireball. It creates an illusion of a fireball. RAW it does not work, but this was not posted in the rules forum.
You tap energy from the Plane of Shadow to cast a quasi-real, illusory version of a sorcerer or wizard evocation spell of 4th level or lower. Spells that deal damage have normal effects unless an affected creature succeeds on a Will save. Each disbelieving creature takes only one-fifth damage from the attack. If the disbelieved attack has a special effect other than damage, that effect is one-fifth as strong (if applicable) or only 20% likely to occur. If recognized as a shadow evocation, a damaging spell deals only one-fifth (20%) damage. Regardless of the result of the save to disbelieve, an affected creature is also allowed any save (or spell resistance) that the spell being simulated allows, but the save DC is set according to shadow evocation's level (5th) rather than the spell's normal level.
The 20% chance to occur is only for non-damaging effect of which a fireball has none. Fireballs do not automatically counter each other you need to use a counterspell action and cast a fireball. The counterspell action is what actually stops the fireball, not the fireball you cast.
What is really going on when you cast a counterspell is you are using your spell to block the other spell. If you succeed neither spell actually manifests. It only works because the spells are identical. Shadow evocation does not becomes a fireball until the spells is actually manifests. The only thing shadow evocation can counterspell is itself.