Winter

When Winter enters the battlefield, sacrifice all
army creatures you control, and create X 0/0
black Zombie Army creature tokens, where X is
the total power of the sacrificed armies. Then put
a +1/+1 counter on each of your Armies.
Whenever you amass, you may choose to proliferate instead.
"A raven came from the citadel... A white raven...
Winter is here."