Signature Spell( Forgotten Realms Campaign Setting, p. 37)
You are so familiar with a mastered spell that you can convert other prepared spells into that spell.
Spell Mastery (PH) ,
Each time you take this feat, choose a spell you have mastered with Spell Mastery. You may now convert prepared arcane ' spells of that spell's level or higher into that signature spell, just as a good cleric spontaneously casts prepared spells as cure spells.
You may gain this feat multiple times. Each time you take the feat, it applies to a new mastered spell.