Maelstrom Nexus is a mythic Enchantment card from Dominaria United Commander (DMC), collector number 159. Current market price: $1.94, TCGplayer market prices via Scryfall.
The first spell you cast each turn has cascade. (When you cast your first spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.)