Hidden Within The Wall

When a creature would enter the battlefield under
your control you may instead put it onto the battlefield facedown under this card. This card
can block with a base power and toughness of
0/10 and has defender. When this card blocks you
may pay {4} if you do the creature under this card enters the battlefield and gains +X,+X where X is
that creatures power.
(You May swap creatures out from this card by returning the card under this one to your hand)