Quandrix, the Proof is a mythic Legendary Creature — Elder Dragon card from Secrets of Strixhaven (SOS), collector number 218. Current market price: $1.26 (foil $1.78), TCGplayer market prices via Scryfall.
Flying, trample Cascade (When you cast this 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.) Instant and sorcery spells you cast from your hand have cascade.