Blightfire Leviathan

Swampwalk
Whenever Blightfire Leviathan deals combat
damage to a player, that player sacrifices a
creature.
At the beginning of your upkeep, you may pay X
life, where X is the number of creatures in all graveyards. If you do, put X +1/+1 counters on Blightfire Leviathan.
The stench of death and decay heralds the approach of
the Blightfire Leviathan. Few can survive its wrath,
and fewer still wish to.