Cheapest priced copy (Theros Promos): $0.49 at Card Kingdom, foil, near mint, their price list as of .
Ember Swallower 
Creature — Elemental · 4/5 · Rare · Theros · #120
{5}{R}{R}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, each player sacrifices three lands of their choice.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros Promos | 120★ | Rare | — | $0.49 |
| Theros | 120 | Rare | $0.49 | $0.49 |
4 rulings from Wizards of the Coast
- 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.
- 15 September 2013When the last ability resolves, the active player will choose three lands they control, then each other player in turn order will do the same. Then all of those lands are sacrificed at the same time.
- 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 2013Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.