Cathar's Shield is a common Artifact — Equipment card from Eldritch Moon (EMN), collector number 192. Current market price: $0.27 (foil $1.21), TCGplayer market prices via Scryfall.
Equipped creature gets +0/+3 and has vigilance. Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)