Windriddle Palaces is a common Plane — Belenon card from Planechase Anthology Planes (OPCA), collector number 85. Current market price: $5.06, 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.