Misleading Song( Races of Stone, p. 142)
You can channel the power of your bardic music to temporarily increase the power of your illusion spells.
Spell Focus (PH) (illusion) , Perform (any one) 5 ranks, bardic music,
As you cast an illusion spell, you can spend one daily use of your bardic music ability to increase the caster level and the saving throw DC of that spell by 1. These bonuses stack with those provided by other feats, such as Spell Focus.