Uncontrolled Plague

At the beginning of your end step add a plague
counter onto Uncontrolled Plague, then all
creatures get X -1/-1 counters put on them where
X is the number of plague counters on
Uncontrolled Plague.
At the beginning of each players upkeep, if there
are no creatures on the battlefield, each player
takes X damage where X is the number of plague
counters on Uncontrolled Plague, then sacrifice it.