Kokusho, the Silent Shadow

Ninjutsu {2}{b}{b}({2}{b}{b}, Return an unblocked attacker you control to hand: Put this card from your hand onto the battlefield tapped and attacking.)
Whenever Kokusho, the Silent Shadow deals combat damage to a player, that player discards a card and sacrifices a creature.
{4}{b}{b}: Return Kokusho from your graveyard to your hand. Activate this ability only if you control two or more Ninja creatures.