Strixhaven Stadium is a rare Artifact card from Strixhaven: School of Mages (STX), collector number 259. Current market price: $0.38 (foil $0.43), TCGplayer market prices via Scryfall.
{T}: Add {C}. Put a point counter on this artifact. Whenever a creature deals combat damage to you, remove a point counter from this artifact. Whenever a creature you control deals combat damage to an opponent, put a point counter on this artifact. Then if it has ten or more point counters on it, remove them all and that player loses the game.