Deathgaze Demon

Flying, deathtouch.
Whenever a creature an opponent control dies, Deathgaze Demon gains +1/+1 until end of turn.
{4}{b}{b}: Monstrosity 3. (If this creature isn't monstrous, put 3 +1/+1 counters on it and it becomes monstrous)
When Deathgaze Demon becomes monstrous, destroy all nonblack creatures.