Ember Swallower

Oracle Text

5RR: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)

When Ember Swallower becomes monstrous, each player sacrifices three lands.

Card Rulings

9/15/2013 When 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.
9/15/2013 Once a creature becomes monstrous, it can’t become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
9/15/2013 Monstrous 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.
9/15/2013 An ability that triggers when a creature becomes monstrous won’t trigger if that creature isn’t on the battlefield when its monstrosity ability resolves.