Korac, Stygian Scourge

Korac has tribute 2 for each opponent you have.
(As this creature enters the battlefield, each opponent
may put two +1/+1 counters on it.)
When Korac enters the battlefield, each
opponent that paid tribute sacrifices two
creatures.
When Korac leaves the battlefield, each
opponent that didn't pay tribute sacrifices each
creature they control.