Quandrix, the Proof is a mythic Legendary Creature — Elder Dragon card from Secrets of Strixhaven Promos (PSOS), collector number 218p. Current market price: $3.89 (foil $6.46), 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.