Cheapest priced copy (Tarkir: Dragonstorm Promos): $0.49 at Card Kingdom, near mint, their price list as of .
Thunder of Unity 
Enchantment — Saga · Rare · Tarkir: Dragonstorm Promos · #231p
(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — You draw two cards and you lose 2 life.
II, III — Whenever a creature you control enters this turn, each opponent loses 1 life and you gain 1 life.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
4 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Tarkir: Dragonstorm Promos | 231p | Rare | $0.49 | $0.49 |
| Tarkir: Dragonstorm Promos | 231s | Rare | — | $0.49 |
| Tarkir: Dragonstorm | 231 | Rare | $0.49 | $0.49 |
| Tarkir: Dragonstorm | 391 | Rare | $0.49 | $0.49 |