Mystic Barrier | Magic: The Gathering
Formats Mystic Barrier is Legal in
Official Oracle Text for Mystic Barrier
Each player may attack only the opponent seated nearest him or her in the last chosen direction and planeswalkers controlled by that player.
Official Rulings for Mystic Barrier
10/17/2013 : Mystic Barrier doesn’t affect creatures that enter the battlefield attacking.
10/17/2013 : In some formats, you may need to disregard teammates sitting between you and the opponent seated nearest you in the chosen direction. In formats like Emperor, other rules may prohibit you from attacking that player.
10/17/2013 : If two Mystic Barriers are on the battlefield with two different chosen directions, you may not attack a player or planeswalker unless that player or the controller of that planeswalker is the opponent seated nearest you in both directions. (This happens most often in two-player games.)