Zant, The Usurper King

Legendary creatures you control get +X/+X, where
X is the number of legendary creatures you control.

Whenever you cast a legendary spell from your
hand, exile cards from the top of your library until
you exile a legendary nonland card with lesser mana
value. You may cast that card without paying its
mana cost. Put the rest on the bottom of your
library in a random order.