Tenacity is a uncommon Instant card from Shadows over Innistrad (SOI), collector number 42. Current market price: $0.07 (foil $0.22), TCGplayer market prices via Scryfall.
Creatures you control get +1/+1 and gain lifelink until end of turn. Untap those creatures. (Damage dealt by those creatures also causes you to gain that much life.)