If a prepare spell with one or more targets has no legal targets when it tries to resolve, it won't resolve and none of its effects will happen. Since the spell was cast, the associated permanent will still not be prepared.
Inspired Skypainter // Maestro's Gift
Only the current controller of a prepared creature can cast the copy of its prepare spell in exile. It doesn't matter who owns that creature or who controlled it when it became prepared.
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. April Prime
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
