Arni Slays the Troll is a uncommon Enchantment — Saga card from Kaldheim (KHM), collector number 201. Current market price: $0.29 (foil $0.27), TCGplayer market prices via Scryfall.
(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.) I — Target creature you control fights up to one target creature you don't control. II — Add {R}. Put two +1/+1 counters on up to one target creature you control. III — You gain life equal to the greatest power among creatures you control.