Zangetsu, the Inner Blade

Bankai (creatures with 2 or fewer colours in their
mana costs cannot have more than one Bankai
attached to them)
Whenever enchanted creature kills a creature an
opponent controls, draw a card and put a +1/+1
counter on enchanted creature.

If this card was put into your hand from anywhere
other than your library, you may destroy target
creature upon casting this spell.