Ngyr-Korath, the Dream Death

Whenever Ngyr-Korath, the Dream Death enters the battlefield from anywhere other than exile, exile
it.
When Ngyr-Korath enters the battlefield or
attacks, target player sacrifices a creature.
{2}{b}{b}{b}: Return Ngyr-Korath from exile to the battlefield tapped. Activate this ability only if
three or more creatures died this turn.