Sentinel's Mark is a uncommon Enchantment — Aura card from Ravnica Allegiance (RNA), collector number 20. Current market price: $0.19 (foil $0.50), TCGplayer market prices via Scryfall.
Flash Enchant creature Enchanted creature gets +1/+2 and has vigilance. Addendum — When this Aura enters, if you cast it during your main phase, enchanted creature gains lifelink until end of turn.