Naglfar, Ship of Nails

Naglfar, Ship of Nails enters the battlefield with ten crew counters.
Whenever Naglfar, Ship of Nails attacks or blocks, you may remove two crew counters, if you do, create two 2/2 black Zombie creature tokens.
{3}{b}, {t}, Sacrifice a creature: Destroy target creature and put crew counters on Naglfar, Ship of Nails equal to the destroyed creature's power.