Berserk is a mythic Instant card from From the Vault: Exiled (V09), collector number 2. Current market price: — (foil $18.20), TCGplayer market prices via Scryfall.
Cast this spell only before the combat damage step. Target creature gains trample and gets +X/+0 until end of turn, where X is its power. At the beginning of the next end step, destroy that creature if it attacked this turn.