Leyline of the Ancients

If Leyline of the Ancients is in your opening
hand you may begin the game with it in play
and it gains the following ability in addition
to its other abilities: Whenever a creature
enters the battlefield under an opponent's
control put X -1/-1 counters on it, where X is
the number of creatures that player controls.
Whenever a creature enters the battlefield under
your control put X +1/+1 counters on it where
X is the number of creatures you control.