Final Showdown is a mythic Instant card from Outlaws of Thunder Junction Promos (POTJ), collector number 11p. Current market price: $6.31 (foil $6.41), TCGplayer market prices via Scryfall.
Spree (Choose one or more additional costs.) + {1} — All creatures lose all abilities until end of turn. + {1} — Choose a creature you control. It gains indestructible until end of turn. + {3}{W}{W} — Destroy all creatures.