Quandrix, the Proof is a mythic Legendary Creature — Elder Dragon card from Secrets of Strixhaven (SOS), collector number 286. Current market price: $4.42 (foil $6.65), 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.