One with the Multiverse is a mythic Enchantment card from Duskmourn: House of Horror Commander (DSC), collector number 121. Current market price: $2.16, TCGplayer market prices via Scryfall.
You may look at the top card of your library any time. You may play lands and cast spells from the top of your library. Once during each of your turns, you may cast a spell from your hand or the top of your library without paying its mana cost.