Mirror Box is a rare Artifact card from Kamigawa: Neon Dynasty Promos (PNEO), collector number 250p. Current market price: $7.96 (foil $10.95), TCGplayer market prices via Scryfall.
The "legend rule" doesn't apply to permanents you control. Each legendary creature you control gets +1/+1. Each nontoken creature you control gets +1/+1 for each other creature you control with the same name as that creature.