Tragedy of the Commons

At the beginning of each player's upkeep, that
player may create a 2/2 white and black Sheep
creature token. If the total number of creatures
on the battlefield exceeds 10, sacrifice Tragedy
of the Commons. Then each player sacrifices
half of their XXX, rounded down.
"In the pursuit of individual gain,
the collective suffers. What once was
lush and shared, now lies barren
and divided."