Mirror Shield is a uncommon Artifact — Equipment card from Theros Beyond Death (THB), collector number 234. Current market price: $0.46 (foil $0.74), TCGplayer market prices via Scryfall.
Equipped creature gets +0/+2 and has hexproof and "Whenever a creature with deathtouch blocks or becomes blocked by this creature, destroy that creature." Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)