Flusterstorm is a mythic Instant card from Secrets of Strixhaven Mystical Archive (SOA), collector number 83. Current market price: $20.99, TCGplayer market prices via Scryfall.
Counter target instant or sorcery spell unless its controller pays {1}. Storm (When you cast this spell, copy it for each spell cast before it this turn. You may choose new targets for the copies.)