Arvinox, the Mind Flail is a mythic Legendary Enchantment Creature — Horror card from Duskmourn: House of Horror Commander (DSC), collector number 130. Current market price: $0.35, TCGplayer market prices via Scryfall.
Arvinox isn't a creature unless you control three or more permanents you don't own. At the beginning of your end step, exile the bottom card of each opponent's library face down. For as long as those cards remain exiled, you may look at them, you may cast permanent spells from among them, and you may spend mana as though it were mana of any color to cast those spells.