Unchecked Plague

When Unchecked Plague enters the battlefield,
put a decay counter on target creature.
At the beginning of each players upkeep, that
player puts a Decay counter on X target
creatures that does not have Decay or a decay
counter. X is equal to the number of creatures
with decay or decay counters on them.
If there are no creatures on the battlefield,
destroy Unchecked Plague.