Believable (3.5e Feat)
From D&D Wiki
You make all the spells from the School of Illusions more real.
Benefit: You gain +2 on the Difficulty Class for all Illusion spells, and Illusion spells only.
Special: This feat stacks with all the other feats that increase the Difficulty Class, such as Spell Focus, but it does not stack with itself and can only be taken once.