Windriddle Palaces is a common Plane — Belenon card from Planechase 2012 Planes (OPC2), collector number 39. Current market price: $4.83, TCGplayer market prices via Scryfall.
Players play with the top card of their libraries revealed. You may play lands and cast spells from the top of any player's library. Whenever chaos ensues, each player mills a card.