Source: Player's Guide to Faerûn
You are so familiar with a mastered spell that you can convert other prepared spells into that spell.
Prerequisite: Spell Mastery.
Benefit: Choose one spell that you have mastered with the Spell Mastery feat as your signature spell. You may now convert prepared arcane spells of that spell's level or higher into your signature spell, just as a good cleric can spontaneously cast prepared spells as cure spells.
Special: You can gain Signature Spell multiple times. Each time you take the feat, it applies to a different mastered spell.