Oath of Kaya is a rare Legendary Enchantment card from War of the Spark (WAR), collector number 209. Current market price: $0.34 (foil $0.47), TCGplayer market prices via Scryfall.
When Oath of Kaya enters, it deals 3 damage to any target and you gain 3 life. Whenever an opponent attacks a planeswalker you control with one or more creatures, Oath of Kaya deals 2 damage to that player and you gain 2 life.