Arni Metalbrow is a rare Legendary Creature — Human Berserker card from March of the Machine: The Aftermath (MAT), collector number 200. Current market price: — (foil $2.07), TCGplayer market prices via Scryfall.
Whenever a creature you control attacks or enters attacking, you may pay {1}{R}. If you do, you may put a creature card with mana value less than that creature's mana value from your hand onto the battlefield tapped and attacking.