Keku's Primordial Gift

(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.)
Exile the top 10 cards of your library face-down (separate from other exiled cards you may have).
Until there are no cards in this pile, you have "Whenever you would draw a card, you may
instead choose to flip a card from the cards exiled
with this spell face-up. If you do, you may cast
that card without paying it's mana cost until end
of turn.