Planar Chaos 
Enchantment
At the beginning of your upkeep, flip a coin. If you lose the flip, sacrifice this enchantment. Whenever a player casts a spell, that player flips a coin. If they lose the flip, counter that spell.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL