Since 2nd edition has generic mages and specialist illusionists I'd say for my own personal usage that penalty per spell lvl of illusions cast by an illusionist would be doubled.Being an old Bard's Tale fan I always loved toying with illusion/phantasm spells. Something that we came up with was allowing a save vs. Intelligence per NPC facing the spell in questions. Generally a lvl 1 illusion was even steven but each level beyond that incurred a -1 penalty to the Save. For each player in the group that "Disbelieve" the illusion and shouted such for other players in the group there would be a +1 to the save. A player could also forfeit all actions on a round to attempt to disbelieve the illusion. For each round of uninterrupted concentration a Save vs Int would be allowed gaining a cumulative +1 to the save each round after the first (i.e. Johny attempts to disbelieve a 1st lvl illusion but keeps failing to do so and is currently at rd 3 still attempting. He would gain a +2 to roll vs int is he is still activively trying to do so). Of course if the character was struck/spelled/etc during that round he'd have to start all over. Later when I discovered 2nd edition and saw intelligence awarded immunity to certain illusion based on your int I was rather pleased and included that as well.
Naturally we came up with several versions of illusion monsters of various spells levels as well. It was kind of complex but only as far as illusions went.
General Notes on Disbelieving Illusions:
Generally any number of illusions may be disbelieved in a single round if cast from generic mages. If cast from Illusionists refer to the table below.
While fighting, spellcasting, or performing some action other than "Disbelieving" will allow a save vs. Intelligence only "ONCE" during the casting of said illusion. If the illusion is disbelieved by that particalar PC/NPC then it will have no affect on him/her. Those that fail will percieve the illusion to be real unless they actively attempt to "Disbelieve" it.
The only exception to this is if one or more fellow members see through the illusion right off the bat and convey their information to others that did not see through the illusion. In this case, they are allowed another Saving Throw vs. Intelligence at the modified group modifier below:
Bonus to group Disbelieving:
0 Members convey: +0 Save vs. Int
1 Member conveys: +1 Save vs. Int
2 Members convey: +2 Save vs. Int
3 Members convey: +3 Save vs. Int, etc....
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Generic Mage Illusions:
Cantrip: +1 Save vs. Int
1st lvl: +0 Saves vs. Int
2nd lvl: -1 Saves vs. Int
3rd lvl: -2 Saves vs. Int
4th lvl: -3 Saves vs. Int
5th lvl: -4 Saves vs. Int
6th lvl: -5 Saves vs. Int
7th Lvl: -6 Saves vs. Int
8th Lvl: -7 Saves vs. Int
9th Lvl: -8 Saves vs. Int
10th Lvl: -9 Saves vs. Int
Illusionist Illusions:
Cantrip: +0 Saves vs. Int
1st lvl: -1 Saves vs. Int
2nd lvl: -2 Saves vs. Int
3rd lvl: -4 Saves vs. Int
4th lvl: -6 Saves vs. Int
5th lvl: -8 Saves vs. Int
6th lvl: -10 Saves vs. Int
7th Lvl: -12 Saves vs. Int
8th Lvl: -14 Saves vs. Int
9th Lvl: -16 Saves vs. Int
10th Lvl: -18 Saves vs. Int
Bonus to Intelligence Saves while "Disbelieving"
Round 1: +1
Round 2: +2
Round 3: +3
Round 4" +4 etc....
*Note must be uninterrupted concentration or the bonus will drop back to +1 and will begin anew. Character can take no other action while "Disbelieving"
