Cheapest priced copy (War of the Spark): $0.79 at Card Kingdom, near mint, their price list as of .
Oath of Kaya 
Legendary Enchantment · Rare · War of the Spark · #209
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.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.79 to $2.29 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| War of the Spark Promos | 209p | Rare | $1.29Out of stock | $1.99 |
| War of the Spark Promos | 209s | Rare | — | $2.29Out of stock |
| War of the Spark | 209 | Rare | $0.79 | $1.49 |
2 rulings from Wizards of the Coast
- 3 May 2019If an opponent attacks more than one planeswalker you control, the last ability of Oath of Kaya triggers once for each of those attacked planeswalkers.
- 3 May 2019If the target is an illegal target by the time the first ability of Oath of Kaya tries to resolve, the ability doesn’t resolve. You won’t gain 3 life.