Oath of Ajani is a rare Legendary Enchantment card from Aether Revolt (AER), collector number 131. Current market price: $0.81 (foil $1.47), TCGplayer market prices via Scryfall.
When Oath of Ajani enters, put a +1/+1 counter on each creature you control. Planeswalker spells you cast cost {1} less to cast.