Planar Chaos is a uncommon Enchantment card from The List (PLST), collector number JUD-97. Current market price: $7.00, TCGplayer market prices via Scryfall.
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.