Cheapest priced copy (Tarkir: Dragonstorm Commander): $0.99 at Card Kingdom, near mint, their price list as of .
Dragonlord Atarka 
Legendary Creature — Elder Dragon · 8/8 · Mythic · Dragons of Tarkir · #216
Flying, trample
When Dragonlord Atarka enters, it deals 5 damage divided as you choose among any number of target creatures and/or planeswalkers your opponents control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.99 to $22.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 216 | Mythic | $1.79 | $16.99 |
| 30th Anniversary Play Promos | 23 | Mythic | — | $22.99 |
| Dragons of Tarkir Promos | 216s | Mythic | — | $5.49 |
| The List | DTK-216 | Mythic | $5.99 | — |
| Tarkir: Dragonstorm Commander | 285 | Mythic | $0.99 | — |
2 rulings from Wizards of the Coast
- 25 February 2015If some of the targets are illegal targets as the ability tries to resolve, the original division of damage still applies but no damage is dealt to the illegal targets. If all targets are illegal, the ability doesn’t resolve.
- 25 February 2015You choose how many targets the ability has and how the damage is divided as you put the ability on the stack. Each target must receive at least 1 damage.