Arni Brokenbrow is a rare Legendary Creature — Human Berserker card from Kaldheim Promos (PKHM), collector number 120p. Current market price: $0.22 (foil $0.37), TCGplayer market prices via Scryfall.
Haste Boast — {1}: You may have Arni's base power become 1 plus the greatest power among other creatures you control until end of turn. (Activate only if this creature attacked this turn and only once each turn.)