Oath of Ajani is a rare Legendary Enchantment card from Aether Revolt Promos (PAER), collector number 131s. Current market price: — (foil $3.41), 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.