Berserk is a mythic Instant card from Conspiracy: Take the Crown (CN2), collector number 175. Current market price: $13.27 (foil $30.99), 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.