Ajani's Aid is a rare Enchantment card from Aether Revolt (AER), collector number 188. Current market price: $0.24, TCGplayer market prices via Scryfall.
When this enchantment enters, you may search your library and/or graveyard for a card named Ajani, Valiant Protector, reveal it, and put it into your hand. If you search your library this way, shuffle. Sacrifice this enchantment: Prevent all combat damage a creature of your choice would deal this turn.