Talfirian Song

Heighten Spell (PH) , Bardic music class feature, Tethyrian human,

By expending uses of your bardic music, you can heighten your illusion spells without using higher-level spell slots. For every daily use of your bardic music you expend during the casting of a spell, you can increase the effective level of that spell by +1. For example, you could cast rainbow pattern and expend four uses of your bardic music to make that rainbow pattern effectively an 8th-level spell (increasing the save DC by +4). You can't raise a spell above 9th level with this feat.

By expending uses of your bardic music, you can heighten your illusion spells without using higher-level spell slots. For every daily use of your bardic music you expend during the casting of a spell, you can increase the effective level of that spell by +1. For example, you could cast rainbow pattern and expend four uses of your bardic music to make that rainbow pattern effectively an 8th-level spell (increasing the save DC by +4). You can’t raise a spell above 9th level with this feat.

You can use the power of your bardic music to enhance your Illusion spells.