Demogorgon

Menace, Trample, Double Strike
Whenever Demogorgon attacks, defending player sacrifices two creatures.
{10}{b}{b}{b}{b}{b} Monstrosity 10 (If this creature isn't monstrous, put 10 +!/+1 counters on it and it becomes monstrous.)
If Demogorgon is monstrous, it gains Indestructible and "At the beginning of every upkeep, put a +1/+1 counter on this creature."