Intelligence
It makes sense that a highly intelligent wizard would be able to create more
effective illusions. It also makes sense that subjects with low intelligence
would be less likely to disbelieve an illusion. Therefore, if the caster's
Intelligence is exceptionally high (18 or more), the subject's saving throw might be
modified by -1. If the subject's Intelligence is exceptionally low (7 or less),
his saving throw might be modified by -1. A -2 modifier is possible if both the
caster's Intelligence is high and the subject's Intelligence is low.
Remember also that characters with exceptionally high Intelligence are
automatically immune to low-level illusions. These immunities are summarized on Table
4 of the Player's Handbook.
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