Mourner's Shield is a uncommon Artifact card from Mirrodin (MRD), collector number 209. Current market price: $0.16 (foil $0.38), TCGplayer market prices via Scryfall.
Imprint — When this artifact enters, you may exile target card from a graveyard. {2}, {T}: Prevent all damage that would be dealt this turn by a source of your choice that shares a color with the exiled card.