Vanguard's Shield is a common Artifact — Equipment card from Avacyn Restored (AVR), collector number 223. Current market price: $0.13 (foil $0.36), TCGplayer market prices via Scryfall.
Equipped creature gets +0/+3 and can block an additional creature each combat. Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)