Ancalagon the Black

Flying, Trample, Fear
{10}{b}{b}{b}{b}{b}{b}{b}: Decimate. (Destroy three target creatures.)
{10}{b}{b}{b}{b}: Monstrosity 4. (If this creature isn't monstrous, put four +1/+1 counters on it and it
becomes monstrous.)
All creatures on the battlefield get a -1/-1 counter on it.