In a Multiplayer Game, player A casts Master Warcraft during my (player B) turn, to try to force me to attack my good friend player C with all my creatures. Player A, being a coward, is hiding behind a ghostly prison. Can he do that? Or is the power of friendship stronger? Also, would the situation change if he were hiding behind a Blazing Archon instead?
Master Warcraft allows player A to choose which creatures attack this turn, but not which opponent or opponent’s planeswalker each creature will attack. That choice remains up to you.
With Ghostly Prison on the battlefield under A’s control, you can still choose to have the creatures attack A, but only if you can afford to pay the {2} for each creature you want to send at A.
If A controls Blazing Archon, you can’t have any of the creatures attack A, but they can still attack planeswalkers A controls, if there are any.