Harmonized Trio // Brainstorm

As an effect causes a creature with a prepare spell to become prepared (including effects that state that a creature "enters prepared"), that creature's controller creates a copy of that creature's prepare spell in exile. That copy remains in exile for as long as that permanent is on the battlefield and is prepared. That creature's controller may cast that copy as long as it remains in exile. As they cast it, that creature stops being prepared.

If an effect instructs you to choose a card name, you may choose the alternative prepare spell's name. Consider only the alternative characteristics to determine whether that is an appropriate name to choose.

If an effect causes a creature to become unprepared, a prepared creature stops being prepared and the associated copy of its prepare spell in exile ceases to exist. If it wasn't prepared at that time, nothing happens.