The Counterspell focused arcane school from the Advanced Player's Guide grants the Disruption ability, which allows the abjurer to touch an enemy spellcaster and force them to make a concentration check to cast spells.
I wanted to make this ability have a stronger effect on the target, so I thought of an arcane discovery that could go with it.
Gastric Disruption
When a spellcaster fails a concentration check while under the influence of your Disruption ability, they must make a Fortitude save. If they fail, they are nauseated for 1d4+1 rounds; if they succeed, they are merely sickened for 1 round.
What sort of prerequisites would be appropriate for this discovery?
I think it's too strong, especially considering that it modifies a 1st level ability that has a fairly large number of uses per day. A 1d4+1 round nauseation totally takes someone out of the fight.
If it's an Arcane Discovery then they need to pay a feat for it, and since it's a melee touch attack you're putting your neck on the line to deliver it. I think that's entirely reasonable, especially given that Abjuration is one of the weakest specialist schools to begin with.
The spell stinking cloud also causes 1d4+1 rounds of nauseation, plus however long the multiple targets stay in the cloud. So, doing it to one target doesn't seem quite that powerful. However, it might be better to limit the discovery to 3/day so that a high Int score doesn't make it too powerful.
Also, giving it a prerequisite of caster level 3 would seem right since stinking cloud is 3rd level and a single-target version of it would probably be 2nd level.
I don't think you'd even need to bother with a prerequisite or limited usage per day. The effect is circumstantial to begin with and has a high chance of failure. It only works on spellcasters, and only if they cast a spell, and even then they need to fail a concentration check, and even then they get a save to mostly negate the effects. This is much worse than Stinking Cloud which immediately and unconditionally forces a save. Given that its duration will be only 1 round until you hit 4th level this means there's a high chance the effect will be resisted entirely.
Hmmm... the necromancer's grave touch ability causes a living creature to become shaken for the duration without a saving throw, and the only difference between shaken and sickened is that sickened imposes a -2 penalty on weapon damage rolls as well -- not a big deal for a spellcaster.
Perhaps the ability should cause the touched spellcaster to become sickened automatically and cause them to become nauseated if they fail the concentration check. In that case, perhaps a Fortitude save to reduce the nausea from 1d4+1 rounds to 1 round only.