Boxing Ring is a rare Artifact card from New Capenna Commander (NCC), collector number 91. Current market price: $1.14 (foil $1.63), TCGplayer market prices via Scryfall.
Whenever a creature you control enters, it fights up to one target creature you don't control with the same mana value. {T}: Create a Treasure token. Activate only if you control a creature that fought this turn.