Cheapest priced copy (Duel Decks: Heroes vs. Monsters): $0.99 at Card Kingdom, foil, near mint, their price list as of .
Polukranos, World Eater 
Legendary Creature — Hydra · 5/5 · Mythic · Theros · #172
{X}{X}{G}: Monstrosity X. (If this creature isn't monstrous, put X +1/+1 counters on it and it becomes monstrous.)
When Polukranos becomes monstrous, it deals X damage divided as you choose among any number of target creatures your opponents control. Each of those creatures deals damage equal to its power to Polukranos.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.99 to $3.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Heroes vs. Monsters | 43 | Mythic | — | $0.99 |
| The List | THS-172 | Mythic | $0.99 | — |
| Theros | 172 | Mythic | $0.99 | $3.49 |
7 rulings from Wizards of the Coast
- 15 September 2013If some, but not all, of the ability's targets become illegal, you can't change the division of damage. Damage that would've been dealt to illegal targets simply isn't dealt.
- 15 September 2013Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
- 15 September 2013You choose the division of damage as you put the ability on the stack, not as it resolves. Each target must be assigned at least 1 damage. In multiplayer games, you may choose creatures controlled by different opponents.
- 15 September 2013The value of X in Polukranos's last ability is equal to the value chosen for X when its activated ability was activated.
- 15 September 2013As Polukranos's triggered ability resolves, Polukranos deals damage first, then the target creatures do. Although no creature will die until after the ability finishes resolving, the order could matter if Polukranos has wither or infect.
- 15 September 2013Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
- 15 September 2013An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.