Nova Prime

Whenever you cast your first spell each turn,
exile the top X cards of your library, where X is
that spell's converted mana cost. Until end of
turn, you may cast a spell from among them with
a converted mana cost equal to or less than the
first spell you cast without paying its mana cost.
At the beginning of the next end step, put all
cards exiled with this ability on the bottom of
your library in any order. For each land returned
to your library this way, untap that many target
lands you contro