Sunbird's Invocation (PPM20-165P) - Ixalan Promos Foil (Promo Pack)

Whenever you cast a spell from your hand, reveal the top X cards of your library, where X is that spell's mana value. You may cast a spell with mana value X or less from among cards revealed this way without paying its mana cost. Put the rest on the bottom of your library in a random order.