The Ur-Sphinx {6}{W}{U}{B}

Eminence — As long as The Ur-Sphinx is in the command zone or on the battlefield, other Sphinx spells you cast cost {1} less to cast.

Flying

Whenever one or more Sphinxes you control attack, each player mills that many cards. For each player, you may cast a card that player milled this way without paying its mana cost.