If I have 2 opponents, one who attacking me and one who has ghostly prison, and I cast windshaper Planetar, can I make the creature attack the player with ghostly prison if the attacking opponent has mana available?
Yes you can.
By the time Windshaper Planetar's trigger is resolving, the creatures have already attacked and so Ghostly Prison's cost doesn't apply to simply reselecting the defending player.










