Foul Soldier

Exploit (When this creature enters the battlefield, you may sacrifice a creature.)
Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)
When Foul Soldier exploits a creature, you may have target player put the top five cards of their library into their graveyard.