Arni Metalbrow is a rare Legendary Creature — Human Berserker card from March of the Machine: The Aftermath (MAT), collector number 66. Current market price: $0.32 (foil $0.45), 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.