Cheapest priced copy (Commander 2013): $0.99 at Card Kingdom, near mint, their price list as of .
Mystic Barrier 
Enchantment · Rare · Commander 2013 · #18
When this enchantment enters and at the beginning of your upkeep, choose left or right.
Each player may attack only the nearest opponent in the last chosen direction and planeswalkers controlled by that opponent.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2013 | 18 | Rare | $0.99 | — |
4 rulings from Wizards of the Coast
- 17 October 2013If 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.)
- 17 October 2013Mystic Barrier doesn’t affect creatures that enter the battlefield attacking.
- 17 October 2013Mystic Barrier affects only what players and planeswalkers each player may attack with creatures they control. It doesn’t affect what players may be targeted by spells or abilities or other interactions.
- 17 October 2013In 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.