
When this creature enters, you may sacrifice a creature or land. If you do, put two +1/+1 counters on this creature.
Having no interest in the greater conflict, the trolls continue to prey upon whatever wanders into their domain.
Illustrated by Greg Staples · Oath of the Gatewatch · #133
2016-01-22
Players can respond to the enters-the-battlefield ability (notably, while Harvester Troll is still 2/3), but once that ability starts resolving, it’s too late for any player to respond. You don’t choose which creature or land you’re sacrificing, if any, until the ability resolves.