Darkrai, Bringer of Nightmares

Whenever Darkrai, Bringer of Nightmares attacks, tap target creature defending player controls and put a nightmare counter on it. That creature doesn't untap during its controller's next untap step.
Whenever a creature you don't control with a nightmare counter dies, put X +1/+1 counters on Darkrai, where X is the number of nightmare counters on that creature.