Survival of the Fittest.

At the beginning of each player's upkeep, they
choose a creature, then sacrifice the rest. Then,
they put X +1/+1 counters onto the chosen
creature, where X is the total power of all
creatures sacrificed this way.
{6}{b}{b/g}: Return target creature from a graveyard
to the battlefield under your control.
Those who survive must prey on the weak.