Na'zuo, Soulless Lich

Whenever you cast a spell from exile from Amass Skeletons 1.
{x}{r}: Exile cards from the top of your library
until you exile X Instants and or Soceries, put
the rest at the bottom of your library in a
random order. Flip a coin for each spell you
exiled this way, if you win you may cast it without
paying its mana cost, if you lose you lose 3 life.