Himself

Okay and guess I understand now, will try to reproduce for my own ends and come back if something goes odd.

Thanks a lot doc! 

So here it goes:

Tested in the following manner: BG2 Irenicus dungeon start, CHARNAME illusionist, Imoen normal mage, Minsc with a weapon that sets all saves to zero.

Results after roughly a 100 casts:
illusionist successfully casted Blindness on Minsc a few times, from memory and from scroll, it also did hit after I changed to Save vs Death.
Imoen didn't hit a single time.

After changing Blindness to Abjuration, the illusionist didn't hit a single time after roughly 50-60 casts, at which point I lost patience

Apparently it works flawlessly and automatically reflects changes made to each spell, so long its tagged within the proper school.

Its just a bit sad that its hardcoded and we cant reproduce or create effects with this... for example a HLA giving Specialists further -1/+1 or for Fighters getting extra + save bonus to resist X school(s), or even attaching such effects to items would be awesome!

Edit: subtledoctor has added an extra about bypassing some limitations that is interesting, here: https://forums.beamd...#Comment_818889

You could try a thousand, or several, more rolls, and try to notice the difference, but this leads to the question of whether the random number generator is fair. I've asked, and... There is one authority who ought to know for sure whether the bonuses apply - Beamdog, if Bioware told them. But I doubt you'll get a simple yes or no from them.

Yup that why I gave up LOL. Too much testing, I'm just going to assume it somehow works.

An old thread says the save bonuses and penalties were not implemented in the original engine.
 
Until someone tests to confirm, the biggest evidence for them having been added in EE is the kit description you posted.  (That text is all new.)
 
If they did implement them, they probably hardcoded the behaviour in the engine, based on the spell school specified in the .spl file and the specialization of the caster and target.
 
There's a new EE effect (opcode #346) that could be used in mods to customize these sorts of things.

Hmm I'm going to try and re-post the same thread in the beamdog forum, maybe we get a DEV or somebody with more accurate/inside info to delve in the subject.

Done - https://forums.beamd...d-a-dev-in-here

Err... Now I'm puzzled  , my splstate.ids doesn't has CHARMED, running un-modded EE latest(2.3), can you experts tell if thats normal?

Thats why I hoped your script could use a junction of LastTargetedBy-LastSummonerof-Myself for your invis cre to automatically target the last creature the spellcaster used a spell on, avoiding conditional circunstances like See. Its hard to give advice on scripting when we don't know exactly what u wanna do and we can't see the script in its entirety. Specially me as I'm dumber than most... :DD