Mirrored Depths is a common Plane — Karsus card from Planechase Anthology Planes (OPCA), collector number 51. Current market price: $3.00, TCGplayer market prices via Scryfall.
Whenever a player casts a spell, that player flips a coin. If the player loses the flip, counter that spell. Whenever chaos ensues, target player reveals the top card of their library. If it's a nonland card, you may cast it without paying its mana cost.