Hoid, Worldsinger

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.
(Jodah, The Unifier)